feat(image-editor): implement PixelForge prototype's placeholder layer ops

[lyfuci]

What

The PixelForge CC 2026 prototype exposes a set of layer operations as placeholders (modals that describe what they would do) or disabled menu items — the "unimplemented but commented" features. Cross-referencing the prototype against toolbox's editor (via a gap-analysis pass) showed the editor already implements most of them (merge down/visible, flatten, smart object, masks, clipping). These four were the genuine gaps, now wired end-to-end:

Feature	What it does
Rasterize Layer	Bakes the selected layer (post effects/clip/opacity) into a plain image-shape layer at the same id/bbox. Reuses the existing composite-ops.rasterizeLayer helper (was internal-only to Convert-to-Smart-Object). Gated for adjustment/filter/mask layers.
Copy / Paste / Clear Layer Style	Moves the fx stack between layers via a useRef clipboard (deep-cloned so gradients/ids don't alias). Pure data move — no render-pipeline change.
Rename Layer	Double-click a layer row (or context-menu Rename) → inline input → patchLayer({ name }). The row shows the custom name when it differs from the dynamic kind label.
Layer Color Tag	Optional PS-style color label (new colorTag on LayerCommon, round-trips through serialize for free). A dot on the row opens a swatch picker. Palette in lib/image-editor/layer-color-tags.ts.

All four are wired into both the Layer menu (MenuBar) and the layer-row context menu. i18n added in en + zh-CN (keys identical across locales).

Notes / deferred

• The gap analysis (4-agent workflow) found the editor is already a near-complete PS clone, so this list is deliberately short and conservative.
• Deferred (stated, not silently dropped): Edit > Fade (the one disabled prototype item — needs before-buffer plumbing; medium effort, can follow up), clipping mask for adjustment/filter layers (large render-pipeline rework), vector mask, and live Info/Navigator panels. These risk regressing the working editor and are better as their own PRs.

Verification

• pnpm typecheck / pnpm lint / pnpm build clean; 388 tests green.
• Headless-Chrome: created a group layer, confirmed all four ops render + function — rename typed and applied, context menu carries all new items, color dot present, Layer menu shows Rasterize + Copy/Paste/Clear Style — zero page errors.

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