feat: combine paint layers + flatten filters

[hm21]

Summary

Adds appearance-preserving combine/flatten operations, so a host app can reduce clutter without changing how the result looks.

Tracking issue in the consuming app: divine-mobile#5834.

1. Combine paint layers (the original ask)

Reduce several selected draw (paint) layers to a single PaintLayer. A true merge to one layer — distinct from grouping (groupId).

bool get canMergeSelectedLayers;    // true when ≥2 non-censor PaintLayers selected
PaintLayer? mergeSelectedLayers();  // merge → one layer, one undo entry, selects it

• PaintLayer now holds List<PaintedModel> items; item stays a back-compat getter/setter to items.first. Single-stroke layers serialize legacy item; merged layers add an items array (minified it); fromMap still reads legacy payloads.
• Pure, unit-tested PaintLayerMergeManager.merge(...) bakes each source layer's offset/scale/rotation/flip/opacity (+ stroke width, erase radius) into a shared frame → identity-transform layer that renders pixel-identical.
• Inserted at the top-most source z-index; originals removed; single undo entry.

2. Flatten filters

bool get canMergeFilters;   // true when ≥2 active filters and none use video-timeline metadata
FilterState? mergeFilters();// flatten the active filter stack into one FilterState, one undo entry

• Exact by construction: ColorFilterGenerator already composes the whole filter+tune stack into one ColorFilter.matrix (mergeColorMatrices), so flattening N filter states reuses that math and is appearance-identical. The filter editor re-reads prior filters as an opaque allMatrices base, so re-editing still works. Gated out when any filter carries video-timeline scheduling.
• Backed by pure mergeFilterStates() / canMergeFilterStates().

Why not text layers / tune adjustments

• Text layers — not possible. A TextLayer is one string + one style + one transform; N independently positioned/rotated/styled texts can't be baked into one without losing appearance or editability. Grouping already covers "handle together".
• Tune adjustments — deliberately skipped. Same color-matrix math would compose exactly, but the tune editor restores each slider by semantic id (el.id == item.id), so flattening to one opaque matrix would reset every slider to 0 on re-open. The re-editability regression isn't worth it. (Happy to add it opt-in if you want the trade-off.)

Tests

flutter test → 507 pass, dart analyze (lib, test, example) clean. New coverage:

• Paint geometry (rotated + flipped + scaled sources map to identical world positions; stroke-width/radius/opacity baking; flattening already-merged sources).
• Paint serialization round-trip (multi-item plain + minified keep N items; legacy single-item still deserializes).
• Paint history (one undo entry; undo restores exact originals) and gating (<2 / censor / mixed non-paint).
• Filter flatten exactness (merged+tune == original stack+tune), gating (<2 / video-timeline), and history/undo.
• Back-compat: existing single-item PaintLayer tests pass; item getter returns items.first.

Housekeeping

• "Combine Paint Layers" button in the layer grouping example.
• Version bumped to 13.2.0 + CHANGELOG entries.