Snap is a magnet button in every graph and vector art tab (#194)

The snap toggle was a checkbox with the word "Snap" next to Fit, and
the vector art tab had its own "Grid" button for the same idea. Both
now share one square icon button, so snapping looks and works the
same wherever it exists.

- SnapToggle is a square toggle button with a magnet icon and no
  text. It lights up blue when snapping is on, like the other toggle
  buttons.
- The magnet is drawn with cubic curves instead of arcs, because the
  Pixi SVG parser draws arcs as jagged edges.
- The vector art tab drops its Grid button and uses the same
  SnapToggle. It still snaps to 8 px, the graph tabs to 24 px.
- SnapToggle accepts anything with setSnapping(on). The vector art
  canvas method is renamed from setSnap to setSnapping for that.
- Every tab exposes the button as __gm.<tab>.snapButton(). Modules
  06, 26 and 36 click through that hook instead of the label "Snap"
  plus 30 px.

Tests:
- ./check.sh: 129 files, 1903 tests (was 128 / 1897)
- 3 new tests in tests/editor/snap-toggle.test.ts, 3 in the icon tests
- GM_STEPS=1-6,26,36,47 fresh: ALL PASSED, 346 checks
- GM_STEPS=47: ALL PASSED, 32 checks (was 29)
- Counter-checks: toggle not switching its target fails 2 unit
  tests, the canvas ignoring the switch fails module 47
- Check total 1188 in README and wiki is computed (1184 + 4), the
  full run is left to CI

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, 1184 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1188 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