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>

src/editor/debugHook.ts

@@ -518,6 +518,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
         }));
       },
       selected: () => (state.selection?.kind === 'shape' ? state.selection.shapeId : ''),
+      selectedIds: () => state.selectedShapeIds,
       /**
        * The shape list in the inspector (#129, stage 4): the order it shows, top first, and where a
        * row sits. A row carries a name that also stands in the tab, so a check cannot click it by text.