M1: measuring, scale tool handles and selection depth (#77)

Measuring (Alt/Option): distances from the selection to the hovered layer,
its parent (over the selection itself or empty space) and ruler guides;
Cmd+Alt for nested layers; containment both ways, overlaps between matching
edges, gaps with dashed extension lines; rotated layers and several selected
layers by their bounds; the measured box outlined. Alt while dragging shows
live distances to the neighbours (or the parent's edges); Shift+Alt reads out
an auto layout frame's padding and gaps. Labels are placed so they never
overlap each other or the size label and stay the same size at every zoom.
Commands: measure.distances, measure.padding.

Scale tool (K) canvas handles: one factor about the Scale panel's anchor
(tool.scaleAnchor, shared by the panel and the canvas; the opposite handle when
the anchor is on the side being dragged), Alt from the centre, flips past the
pivot, rotated layers and lines along their own axes, one undo step per drag,
a live factor in the size label and the pivot marked. scale_node now also
scales min/max sizes, layout grids, guides, fixed grid tracks, tiled images,
paragraph indent and run letter spacing.

Selection: Cmd/Ctrl-marquee selects nested layers, Shift-marquee toggles,
the context menu's Select layer lists every layer under the click
(select.layersAt), outlines are purple for every layer inside components and
instances, hover outlines show with the Scale tool too. Fixed marquees
selecting layers that only overlapped the marquee on one axis.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -374,6 +374,31 @@ and a file keeps the newest 20 autosaves (user versions are never dropped for th
 coordinates: they move with the frame, span only it, are copied with it and snap the layers inside
 it. In the app, a guide dragged out of a ruler and dropped on a frame becomes that frame's guide.
 
+### Measuring, the Scale tool and selecting
+
+- **Measure** (what Alt/Option-hover shows): `measure.distances {ids? (default the selection), to?
+  (a layer; default the selection's parent)}` returns `{lines: [{x0, y0, x1, y1, label, dashed}]}`
+  in page coordinates, between bounding boxes (several layers by their union, rotated ones by
+  their bounds): one box inside the other measures to each edge, boxes apart measure the gaps
+  (`dashed` lines extend a gap line to a box it passes beside), overlapping boxes measure between
+  matching edges. `measure.padding {id? (an auto layout frame or a child)}` returns `{frame, lines}`:
+  the frame's padding and the gaps between neighbouring children (what Shift+Alt-hover shows).
+  With `pointer`, `alt: true` hovers measure (add `cmd` for nested layers, `shift` for padding) and
+  `alt` drags show live distances to the neighbours; `inspect_ui` reports them under
+  `tool.overlay` (`guides`, and `measure_box` for the box measured to).
+- **Scale tool** (`tool.scale`, K): dragging a selection handle with `pointer` scales everything by
+  one factor (sizes and positions whatever the constraints, strokes, effects, corner radii, auto
+  layout spacing, min/max sizes, layout grids, guides, tiled images, text), one undo step per drag,
+  about the anchor point `tool.scaleAnchor {anchor?: tl|t|tr|l|c|r|bl|b|br}` (or, when the anchor
+  lies on the side being dragged, the opposite handle; `alt: true` scales from the centre), with the
+  factor and pivot in `tool.overlay.scale` while dragging. Without
+  dragging, `transform.scale {ids?, factor, cx?, cy?}` does the same numerically.
+- **Select**: `select.layersAt {x, y}` lists every layer under a page point, front-most first with its
+  depth (the context menu's Select layer); `select.set`, `select.children` / `select.parent` (Enter
+  / Shift+Enter), `select.next` / `select.previous` (Tab / Shift+Tab). With `pointer`, `cmd: true`
+  clicks select the nested layer, `cmd` drags from empty canvas select the nested layers they touch,
+  and `shift` drags toggle what they touch.
+
 Tool failures come back as results with `isError: true` and a message; protocol problems (unknown
 tool, malformed JSON, wrong parameter types) are JSON-RPC errors. Nothing crashes the server.