A frame's collision outline can be generated from its pixels (#161)

Drawing an outline by hand for every frame of a sheet is slow, and the
outline mostly follows the opaque part of the image anyway. The texture
tab now traces it: one click, a corner budget, and every Frame
collision collider and tile using that frame blocks with the result.

- "Generate collision..." sits in the texture tab header and is off
  while the texture has no frames.
- The dialog has Max points (default 8, 3 to 64), Solid from ... %
  alpha (default 50) and All frames of the texture. A live line says
  what Generate will write, and Generate stays off when nothing is
  solid enough.
- traceCollision walks each island of solid pixels along the pixel
  edges, then drops the least important corners until at most N are
  left and no stair step remains. A slope becomes one straight edge.
- An outline never crosses itself. Where the image would force that,
  the island's convex hull is taken instead.
- Crumbs under 1 % of the frame area are dropped unless they are the
  only island. Holes are not reproduced.
- Existing outlines are replaced. One frame or all frames is a single
  undo step.
- TextureLibrary hands out the sheet's RGBA data, and a pure function
  cuts a frame's region from it.

Findings:
- The wedge in the e2e fixture gets 4 corners, not 3. Its tip is one
  pixel wide in the image, and that real edge stays while corners are
  still free.

Tests:
- ./check.sh: 131 files, 1941 tests, CHECK EXIT 0 (was 130 / 1931).
- 10 new unit tests in tests/core/trace-collision.test.ts, including
  150 random shapes that must all be valid and within the budget.
- GM_STEPS=42, freshly seeded: ALL PASSED, 20 checks (was 16).
- Three counter-probes bite: no hull fallback, no stair threshold,
  dialog ignoring the alpha threshold.
- Check sum 1202 in README is computed (1198 + 4), CI measures it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1198 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1202 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs