Refactoring round 6: shared sheet canvas, fewer copies (#85)

Recurring cleanup round. Behaviour is unchanged. Duplicated code in
src went from 27 clones with 299 lines to 6 clones with 56 lines
(jscpd, the two identical OFL licence texts not counted).

- SheetCanvas is the new base of the texture, pixel art and vector art
  canvases. It holds background, mask, hint, panning, wheel zoom, fit,
  view save and restore and the screen mapping. Pixel art keeps its
  whole-pixel panning and fixed zoom steps.
- mixColor in src/core/util/color.ts replaces three copies in
  animation, gradient and particles. It clamps t like the particle one.
- fingerprint now lives once in generated.ts, next to the revision
  contract from #185.
- MoveCommand next to RenameCommand is the base of the three reorder
  commands for tile pictures, tilemap layers and vector shapes.
- The two raycast nodes share inputs, settings and filter via rayOf.
- canvasRect() lives once in AssetTabEditor.
- Removed unused code: findPixelArt, isDrawableShape,
  GeneratedPictures.has and SnapToggle.snapping. Tests now check the
  same things through equivalent paths.
- 38 values used only in their own file lost their export.
- Three comments that told history now describe the current state.
  The bundle stayed md5-identical for those edits.
- README lists vector art, pixel art and palette, and links the two
  wiki pages.

Findings:
- PaintTilesCommand.changes is meant to prevent empty undo steps but
  has not been called since #128. The rectangle tool therefore records
  a no-op undo step when it paints the same tile over itself. Left as
  is, because fixing it changes behaviour.
- No test covered middle-click panning in any sheet tab before. 13f
  and 48a now check it.
- The default values of the ray inputs were untested. Length 1000
  could become 10 without a red test. tests/core/raycast.test.ts now
  covers it.
- CI measured 1308 editor checks at d7c0b7c, not the 1307 counted for
  #164. The sum is now 1310 (1308 + 2), computed.

Tests:
- ./check.sh: 144 files, 2143 tests, exit 0 (before 2142).
- e2e 1-16,37,39-43,47,48 fresh: ALL PASSED, 729 checks.
- e2e 1-13,47,48 with the new checks: ALL PASSED, 518 checks.
- Counter-checks, each restored via cp and cmp: halved panning fails
  both new pan checks; unclamped mix fails 1 test, no blue channel 3;
  coarser hash 3; broken reorder undo 2; triggers setting forced on 2;
  bent ray length 1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/graph/nested.ts

@@ -67,7 +67,7 @@ export function fixedPortKeys(def: NodeDef): string[] {
   return [...fixed.map((p) => p.key), ...def.params.map((p) => p.key)];
 }
 
-export function declaredOn(
+function declaredOn(
   def: NodeDef,
   ports: readonly DeclaredPort[],
   side: 'inputs' | 'outputs',