Smart Guides: a tool switched to under a still pointer is hovered at once

The hover's context now includes the active tool (PR review follow-up):
switching to the Selection tool with Alt held and the pointer still
showed no Distance Guides until the pointer moved, since the new tool
had never hovered. A tool switch re-hovers at the pointer whatever the
tool; the rest of the context (canvas, view, document and revision)
re-hovers tools that ask (Tool::wants_rehover).

canvas::tests::distance_guides_follow_what_lies_under_a_still_pointer
adds a tool switch (fails without the rule), a modal dialog with its
backdrop over the pointer, and another document opened under it.

docs/target-app-parity.md

@@ -689,7 +689,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   Panel Options › Locate Selected Objects (beyond Illustrator, off by default): when the last selected object changes on the canvas or by a command, the panel opens the layers and groups around it and scrolls to its row, as Locate Object does, without highlighting the row; when Show Layers Only, a search or the filter leaves the object without a row, the panel opens no rows for it and scrolls as with the option off.
 - **Drawing aids:** Smart Guides and snapping (moving, with the Selection, Group Selection or Direct Selection tool, and resizing by the bounding-box handles with the Selection or Free Transform tool, Shift and Alt too: the dragged corner or side lands on the other objects' anchors and centres ("anchor", "center") or lines up with their edges and centres (a line and "align"); with Smart Guides › Spacing Guides, a moved selection that lines up with nothing along an axis lands as far from its neighbour as two other objects of its row or column are apart, or halfway between its two neighbours, the equal gaps marked; with View › Snap to Grid on, a moved selection lands on the grid instead, by whichever of its edges is nearest a gridline on each axis (the center does not snap), #740, #1816; the Rotate, Scale, Shear and Reflect tools drag from the anchor they grab and snap the pointer the same way, so a corner dragged onto another object's anchor scales exactly onto it, #482; anchors dragged with Direct Selection: the one pressed on lands on other anchors, its own path's that stay put among them, on path segments or in line with them, the others following, Shift keeps 45° steps, and with Smart Guides off Snap to Point pulls it onto anchors; the Artboard tool snaps artboards, and their bleed, to other artboards, bleeds and art, the grid or whole pixels while moving, copying and resizing them, and the Transform panel shows and edits the active artboard's X/Y/W/H; drawn points, #506: hovering shows where the press goes, the point lands on the other objects' anchors, centres and paths ("anchor", "center", "path") or lines up with their anchors, edges and centres ("align"), the Pen's on the path being drawn too, near a construction guide through the anchor before it on that guide (Preferences › Smart Guides › Construction Guides and Angles), and with Shift kept to its 45° step or a square's diagonal it slides along that way into line; the targets are gathered once per document state, from the art in the window, and the anchors, centres and lines near the pointer are found by bisection, so hovering never rescans the document), guides dragged out of the rulers (#451: they snap as moved guides do, Shift to the ruler's ticks; with the Artboard tool they are artboard guides, which run across their artboard only and move, copy and delete with it, `guide.add {artboard}`) that the Selection, Direct Selection and Group Selection tools then pick and drag (#414: the selected guides highlight and move together, Alt copies them, Shift snaps the dragged one to the ticks, otherwise it snaps to pixels, the grid or the edges, centres (so the sides' midpoints) and anchors of the art and the artboards, or with Smart Guides off Snap to Point pulls it onto anchors; Shift-clicked they join the selected art, and Shift-added art keeps them, so guides and art are selected together in either order, Delete then deleting both in one step; the arrow keys nudge them, Delete deletes them, dragging one back onto its ruler deletes it, Lock Guides and Hide Guides leave them alone; `guide.list`, `guide.select`, `guide.move` and `guide.remove` reach agents), a right-click on either ruler or the origin box to pick the document's units (the current one checked; `document.setUnits`, one undo step, #528), and Draw Normal / Behind / Inside modes. Smart Guides: a point drawn with the Pen or a shape tool, and an anchor dragged with Direct Selection, snaps to where paths and ruler guides cross, labelled "intersect" (#1658, part of #1602; a moved selection's own anchors don't).
   A move that Shift keeps level, upright or diagonal stays on that line when it snaps, with the Selection, Group Selection or Direct Selection tool. It slides along the line until the selection lands on the grid or on whole pixels, lines up with the other objects or is spaced evenly with them, or until the line passes an anchor or crosses a ruler guide near the point the selection was grabbed by. A level or upright move snaps along its own axis only.
-  Distance Guides: with Smart Guides on (View › Smart Guides and Preferences › Smart Guides › Invoke Distance Guides, on by default), something selected and the Selection tool, holding Alt/Option over another object shows the distances between the selection's bounds and that object's: on an axis on which they lie apart, one line across the gap between their facing edges; on one on which they overlap, the offsets of their left and right (top and bottom) edges, each line running across the middle of where they overlap, with a dashed line out to the other object's edge when it passes beside it. Over an empty part of an artboard it shows the selection's distances from the artboard's left, right, top and bottom edges. Each line has ticks at its ends and its length in a pill of the smart guide colour, in the General unit; letting Alt go, pressing (a pan or zoom press too), moving off them, leaving the canvas or a floating panel, menu or dialog over the pointer takes them away, and with the pointer still they follow what lies under it as the view scrolls, zooms or turns and as the document or the selection changes. Behaviour from the public [Use Distance Guides for accurate placement](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/use-distance-guides-for-accurate-placement.html) and [Smart Guides options](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/smart-guides-options.html) pages, which describe hovering only: nothing shows while moving or nudging. Tests: `select::tests::alt_over_*`, `select::tests::distance_guides_need_alt_a_selection_and_smart_guides`, `tests_prefs::invoke_distance_guides_measures_the_selection_to_the_object_under_alt`, `canvas::tests::alt_over_another_object_draws_the_distance_guide_label`, `canvas::tests::distance_guides_go_when_the_hover_ends`, `canvas::tests::distance_guides_follow_what_lies_under_a_still_pointer`.
+  Distance Guides: with Smart Guides on (View › Smart Guides and Preferences › Smart Guides › Invoke Distance Guides, on by default), something selected and the Selection tool, holding Alt/Option over another object shows the distances between the selection's bounds and that object's: on an axis on which they lie apart, one line across the gap between their facing edges; on one on which they overlap, the offsets of their left and right (top and bottom) edges, each line running across the middle of where they overlap, with a dashed line out to the other object's edge when it passes beside it. Over an empty part of an artboard it shows the selection's distances from the artboard's left, right, top and bottom edges. Each line has ticks at its ends and its length in a pill of the smart guide colour, in the General unit; letting Alt go, pressing (a pan or zoom press too), moving off them, leaving the canvas or a floating panel, menu or dialog over the pointer takes them away, and with the pointer still they follow what lies under it as the view scrolls, zooms or turns, as the document, the tab or the selection changes, and when the Selection tool is switched to. Behaviour from the public [Use Distance Guides for accurate placement](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/use-distance-guides-for-accurate-placement.html) and [Smart Guides options](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/smart-guides-options.html) pages, which describe hovering only: nothing shows while moving or nudging. Tests: `select::tests::alt_over_*`, `select::tests::distance_guides_need_alt_a_selection_and_smart_guides`, `tests_prefs::invoke_distance_guides_measures_the_selection_to_the_object_under_alt`, `canvas::tests::alt_over_another_object_draws_the_distance_guide_label`, `canvas::tests::distance_guides_go_when_the_hover_ends`, `canvas::tests::distance_guides_follow_what_lies_under_a_still_pointer`.
   A move pressed on a path's stroke, or within 6 px of its outline away from its anchors and center, is held by the nearest point of that path, with the Selection, Group Selection or Direct Selection tool, and a straight segment Direct Selection drags is held by its own nearest point. That point is what snaps, so the path lands exactly on a ruler guide (with "align") instead of the pressed spot landing there and the path a few pixels beside it, an offset that showed only zoomed far in (#3701). A press in the middle of a fill still snaps the pressed spot: the public [Align objects with guides](https://helpx.adobe.com/illustrator/desktop/measure-and-align/grids-and-guides/align-graphic-objects-with-guides.html) page has Snap to Point align the point under the pointer, which is placed exactly where the object should meet the guide.
   A bounding-box handle dragged with the Selection or Free Transform tool snaps what it moves as a dragged point snaps (#2054). With View › Snap to Grid on, it snaps to the nearest gridline, and with Snap to Pixel on, it snaps to whole pixels. With Smart Guides off and Snap to Point on, it snaps to the other objects' anchors and the ruler guides within the Snap to Point distance. With Smart Guides and Snap to Point both on, it lines up with the ruler guides as well as the other objects. Shift still keeps the proportions, and Alt still scales about the center.
   A user slice's edge or corner handle dragged with the Slice Selection tool snaps as the Slice tool's drag does. With View › Snap to Grid on, it snaps to the nearest gridline, and with Snap to Pixel on, it snaps to whole pixels. Otherwise, with Smart Guides on, it lands on or lines up with the anchors, centers and edges of the art and the artboards, and with Smart Guides off and Snap to Point on, it snaps to the art's anchors and the ruler guides within the Snap to Point distance. Then a ruler guide within 5 px, shown and unlocked, moves the side or corner onto it. Shift still keeps the proportions, and Alt still resizes about the center.