Smart Guides: Alt over another object or the artboard shows the selection's distances to it

Distance Guides, from the public "Use Distance Guides for accurate
placement" and "Smart Guides options" (Invoke Distance Guides) pages:
with Smart Guides on, something selected and the Selection tool,
holding Alt/Option over another object shows the horizontal and
vertical distances between the selection and it, and over an empty
part of an artboard the selection's distances from the artboard's left,
right, top and bottom edges.

- guides::distance_guides measures the selection's bounds to a box: on
  an axis on which they lie apart, the gap between their facing edges;
  otherwise the offsets of both pairs of matching edges. Each line runs
  across the middle of the overlap on the other axis (else the
  selection's middle), with a dashed line out to the target's edge when
  it passes beside it. An artboard is always measured edge to edge.
  Labels are in the General unit, in the smart guide colour.
- The Selection tool keeps what its Alt hover points at and draws the
  guides while idle; letting Alt go (the canvas re-hovers on a modifier
  change), a press or moving off clears them.
- Overlay::Distance: the line with end ticks and its length in a pill;
  the canvas draws it.
- Preferences › Smart Guides › Invoke Distance Guides (`distanceGuides`,
  on by default), with i18n rows; View › Smart Guides gates it too.
- Hover only, as the pages describe: nothing while moving or nudging.

Tests: select::tests::alt_over_* (gap and edges beside and below,
artboard edges in pt and mm), 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 (frame test of the drawn label and line).

docs/mcp.md

@@ -2721,6 +2721,11 @@ Guides change where a snapped point lands:
 - `spacingGuides` (on by default): a moved selection (Selection tool / `pointer_gesture` with `selection`) snaps so
   the gap to a neighbour matches the equal spacing among other objects nearby; off, only alignment and construction
   guides pull.
+- `distanceGuides` (Invoke Distance Guides, on by default): with Smart Guides on, something selected and the Selection
+  tool hovering with `mods.alt` (a `pointer` event of kind `move`), the canvas measures the selection's bounds to the
+  object under the pointer (the gap between their facing edges on an axis on which they lie apart, else the offsets
+  of their two edges on it) or, over no object, to the left, right, top and bottom edges of the artboard under the
+  pointer: measured lines labelled in the General unit. Off, Alt-hovering shows none.
 
 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