Vector shapes can be combined by union, subtract and more (#187)

Building a shape out of simpler ones meant drawing the result point by
point. The Vector Art tab now selects several shapes at once and
combines them with a boolean operation into one ordinary shape.

- Ctrl/Cmd+click in the tab and Ctrl or Shift+click in the shape list
  add a shape to the selection. Shift stays pan in the canvas.
- A COMBINE block in the inspector offers Union, Subtract, Intersect
  and Exclude. Subtract removes every upper shape from the lowest one.
- The result replaces the lowest selected shape and keeps its name,
  colours, stroke and place in the order. It is one undo step.
- Curves are flattened to 0.2 canvas pixels, a circle gets about 48
  points. Open lines and single shapes cannot be combined, the buttons
  are off and a hint gives the reason. An empty result is rejected.
- Delete now removes every selected shape, and the selection drops
  shapes that no longer exist.
- The geometry comes from clipper2-ts 2.0.1, pinned. It has no
  dependencies and speaks even-odd directly. Only the engine module
  is imported, the bundle grows by 55 kB instead of 110 kB.

Findings:

- clipper2-js was tried and dropped. It pulls in Angular 15 with a
  high npm audit finding and no fix.
- Pixi finds holes by containment, not by even-odd depth. An island
  inside a hole would be drawn as a hole again. Result rings are
  emitted outer ring before its islands, and a test mirrors Pixi's
  rule.
- A depth sort turned out to be dead code in a counter-check and was
  removed. A pixel check in 47h stayed green without the subtract
  and now checks both colours.

Tests:

- ./check.sh: 134 files, 1986 tests, exit 0 (before 133 / 1968)
- npm run build: exit 0
- GM_STEPS=47, freshly seeded: ALL PASSED, 44 checks (before 39)
- 18 new unit tests, 5 new e2e checks in block 47h
- Five counter-checks bite, originals restored and verified with cmp
- Check sum 1222 in README and wiki, calculated (1217 + 5)

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

package-lock.json

@@ -9,6 +9,7 @@
       "version": "0.1.0",
       "dependencies": {
         "@dimforge/rapier2d-compat": "^0.20.0",
+        "clipper2-ts": "2.0.1",
         "pixi.js": "^8.6.0"
       },
       "devDependencies": {
@@ -1633,6 +1634,15 @@
         "node": ">= 16"
       }
     },
+    "node_modules/clipper2-ts": {
+      "version": "2.0.1",
+      "resolved": "https://registry.npmjs.org/clipper2-ts/-/clipper2-ts-2.0.1.tgz",
+      "integrity": "sha512-raIgNMpYN/PFYk/T9iRzaG99rf5nlXBWL1FyAdR3wjEYSzJy+pPfolLH5bCdRqCFQqn/eYUsF1jb3cQEMxmWGw==",
+      "license": "BSL-1.0",
+      "engines": {
+        "node": ">=14.0.0"
+      }
+    },
     "node_modules/cross-spawn": {
       "version": "7.0.6",
       "resolved": "https://registry.npmjs.org/cross-spawn/-/cross-spawn-7.0.6.tgz",