Fix #506: drawing tools snap to Smart Guides as the Selection tool does

Every drawing tool now asks one snapper (guides::DrawSnap) for the points it
places: the Pen's and Curvature tool's anchors, the Pencil's ends, the shape
tools' start and dragged corner or radius (Rectangle, Rounded Rectangle,
Ellipse, Polygon, Star, Line Segment, Arc, Spiral, the grids), the Type tool's
click and frame and a new artboard's corners. Points land on other objects'
anchors, centres and paths ("anchor", "center", "path") or line up with their
anchors, edges and centres ("align"); hovering shows it before the press, the
Pen's guides end its rubber band, and the Pen lands on the path being drawn
too. Shift keeps a point to its 45-degree step (from Constrain Angle) or a
box's diagonal and slides it along that way into line with a target.
Preferences > Smart Guides > Construction Guides and Angles now act: a drawn
point near a guide through the point it leaves from lands on it.

Before, the Pen snapped only its press and showed nothing, and the shape tools
re-gathered targets on every drag step from the document showing their own
preview, so the dragged corner stuck to where the last step left it.

Performance: targets are gathered once per document state (keyed on the
document's uid and revision, now in ToolContext), from the art in the window,
and sorted, so the anchors, centres and lines near the pointer are found by
bisection. Leaving objects out of the targets no longer looks up each node's
ancestry (quadratic in the document's size): one pruned walk does it, which
also speeds up the Selection tool's moves and resizes.

The direction handles' HandleSnap (#494) is now a DrawSnap holding its own
targets (Targets::for_handle) for the drag: one cache, one lookup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -1215,8 +1215,8 @@ off, an empty window, and the Home button or `app.home` still shows the screen)
 default: on, a click away from a panel popped out of the icon column puts it away).
 
 The Smart Guides preferences (Preferences › Smart Guides) apply to `pointer_gesture` with Smart Guides on (the
-default view) and to the mouse; they change what the tools show and how far a target pulls, never where a snapped
-point lands:
+default view) and to the mouse; they change what the tools show and how far a target pulls, and only Construction
+Guides change where a snapped point lands:
 
 - `smartGuideColor` (`#ff3dfc` by default): the colour of the smart guides' lines and labels.
 - `alignmentGuides` (on by default): off, no line is drawn along the edge or centre the art lines up with; the art
@@ -1231,6 +1231,11 @@ point lands:
 - `snappingTolerance` (0–40 px, 4 by default): how near an anchor, edge, centre or artboard edge pulls a drawn point,
   a dragged selection, a bounding-box handle, a ruler guide or an artboard. With Smart Guides off, Snap to Point uses
   `snapToPointTolerance` instead.
+- `constructionGuides` (on by default) and `constructionAngles` (`90° & 45° Angles` by default; also `90° Angles`,
+  `45° Angles`, `60° Angles`, `30° Angles`, `90° & 45° & 30° Angles`): a point drawn near a line at one of those
+  angles through the point it leaves from (the Pen's or Curvature tool's last anchor, a line's start) lands on that
+  line, where a target's alignment line crosses it if one does nearby. "90°" gives the horizontal and the vertical,
+  "45°" the diagonals, "60°" and "30°" the multiples of those angles in between. Off, the point only lines up.
 
 With Smart Guides on, a bounding-box handle dragged with the Selection or Free Transform tool (`mods.shift`
 proportional, `mods.alt` from the centre) lands on another object's anchor or centre (labelled "anchor" or
@@ -1242,7 +1247,26 @@ snap the pointer as drawing tools do, so dragging a corner onto another object's
 {"name":"pointer_gesture","arguments":{"tool":"scale","events":[{"kind":"down","x":100,"y":100},{"kind":"up","x":100,"y":100},{"kind":"down","x":199,"y":198},{"kind":"drag","x":302,"y":262},{"kind":"up","x":302,"y":262}]}}
 ```
 
-Not read yet: Construction Guides and their Angles, and Spacing Guides (the tools draw neither).
+Every point a drawing tool places snaps the same way, through one snapper (#506): the Pen's and Curvature tool's
+anchors, the Pencil's first and last point, the shape tools' start and dragged corner or radius (`rectangle`,
+`roundedRectangle`, `ellipse`, `polygon`, `star`, `lineSegment`, `arc`, `spiral`, `rectangularGrid`, `polarGrid`), the
+Type tool's click and frame, and a new artboard's corners. The point lands on another object's anchor, centre or
+path ("anchor", "center", "path"), else lines up with the anchors, edges and centres of the art and the artboards (a
+line from the target and "align"); the Pen's lands on the path being drawn too, so a click on its first anchor closes
+it exactly. With `mods.shift` the point keeps to its 45° step from the anchor before it (from `constrainAngle`; a box
+dragged keeps its corner on a diagonal) and slides along that way into line with the nearest target. A `move` event
+before the press shows where it would go, as hovering does with the mouse. The targets are the art in the window (all
+of it headless), gathered once per document state; the shape being drawn is never one of them:
+
+```json
+{"name":"pointer_gesture","arguments":{"tool":"rectangle","events":[{"kind":"down","x":202,"y":203},{"kind":"drag","x":330,"y":341},{"kind":"up","x":330,"y":341}]}}
+{"name":"pointer_gesture","arguments":{"tool":"pen","events":[{"kind":"down","x":300,"y":420},{"kind":"up","x":300,"y":420},{"kind":"down","x":151.5,"y":431},{"kind":"up","x":151.5,"y":431}]}}
+```
+
+With a 100 × 100 square at (100, 100), the first starts the rectangle on its corner (200, 200); the second puts the
+Pen's second anchor at (150, 431), in line with the square's centre.
+
+Not read yet: Spacing Guides (the tools draw none).
 
 ## Type preferences