ui(resize): wall handles — half-size wall-with-arrows glyph, grab cursor

Owner request: in the resize tool make the wall-drag handles twice
smaller, replace the circle with a 'wall + two opposite arrows' icon,
drag cursor.

- visible glyph: wall segment with two arrows perpendicular to the
  edge (the drag directions), rotated per wall orientation; accent ink
  over a --hp-bg halo, readable on any plan
- HIT area unchanged: an invisible circle of the original finger-sized
  radius keeps touch targets and the HP-1550-04 hit priority over
  openings (04_handle_wins_hit_test still passes)
- cursor: grab on hover, grabbing while dragging (:active)
- scale-frame corner handles untouched (classic filled circles,
  nwse-resize)
- docs/RESIZE.md: handle appearance updated

docs/RESIZE.md

@@ -19,9 +19,14 @@ before — no handles, no new hit areas.
 ## Mechanism A — wall drag
 
 - Every visible room shows a small handle at the midpoint of every
-  wall (handles for all rooms at once — owner's UX pick; they are
-  finger-sized but unobtrusive, radius derived from `view.w` like
-  `.vacfithandle`).
+  wall (handles for all rooms at once — owner's UX pick). The visible
+  glyph is a compact icon — a wall segment with two arrows pointing
+  perpendicular to it (the directions the wall drags), rotated to the
+  wall's orientation; accent ink over a `--hp-bg` halo. It is half the
+  size of the old circle, but the HIT area is an invisible circle of
+  the original finger-sized radius (derived from `view.w` like
+  `.vacfithandle`), so touch targets did not shrink. Cursor: `grab`,
+  `grabbing` while dragging.
 - Dragging a handle moves the wall along its outward normal; **both
   ends of the edge translate together** (the wall stays parallel to
   itself; adjacent walls stretch/shrink). Works for any polygon