Summary

Promotes the Blur tool from stub to functional. Drag a rectangle on the
canvas; the area inside is blurred at render time using the canvas
`filter: blur(Npx)` primitive.

Modeled after the Mosaic tool — same "sample the underlying composite,
draw it back in place" pattern, just with a Gaussian blur instead of
pixelation. Cheap, non-destructive, undoable.

Implementation

• New `BlurShape` (kind: `'blur'`) with x/y/w/h + radius (default 8 px,
  preview-pixel space). Joins the `Shape` union; round-trips through
  serialize/parse via the existing forwards-compat path.
• `drawing.ts` gets `drawBlurRegion`: `ctx.filter = blur(r * scale)`,
  then `drawImage(underlying, src, dst)` of the rect onto itself. Scale
  applies to the radius so blur looks consistent across preview and
  export.
• `render.ts` snapshots the canvas before any mosaic OR blur layer (same
  underlying-canvas-read need).
• `hit.ts` + `transform.ts`: blur falls into the rect-corner-handle
  case, so move + resize "just work" via the existing pattern.
• `Canvas.tsx`: tool `'blur'` starts a rect-style drag like mosaic.
• `OptionsBar`: blur shares the simple-hint variant with mosaic/mask.
• `ToolsPalette`: new entry next to Mosaic in the Annotation group
  (Aperture icon).
• `LayersPanel`: `annoLabel.blur` for the layer list.

Removes `blur` from `STUB_TOOLS`.

Branched from `main` (independent).

Test plan

• Press the Blur palette button — selected, options bar shows the blur hint.
• Drag a rect on the image — area inside is visibly blurred (default 8 px radius).
• Switch to V, drag a corner handle on the blur layer — region resizes; blur follows.
• Click + drag the blur layer — region moves.
• Cmd+Z undoes; Cmd+Shift+Z redoes.
• Save .json project, reload — blur layer persists.
• Export PNG — blur is baked at source resolution (radius scaled).

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