feat: geometric shapes on the Atlas board (goal 0169 slice 5)

[alicoding]

Summary

• Rectangle, ellipse, and arrow join image/ink as a fourth board-local BoardObject Kind (shape), drawn via the existing drag-to-draw interaction (no new interaction shape needed).
• Fill/stroke/strokeWidth stay live Payload data — never baked into a mirror file the way ink's pixels are — rendering straight from Payload + the already-generic Size field. First real use of SetBoardObjectSize outside a manual resize.
• Zero Go files touched. CreateBoardObject/SetBoardObjectSize/PromoteBoardObject/DeleteBoardObject were already Kind-agnostic since goal 0179 S1 — confirmed by git diff --stat showing no .go files in this diff.
• Fixed a structural bug in AtlasCreationTray.tsx: a single shared anchor ref reused across every mapped drag-to-draw button would have silently mispositioned one tool's own options popover the moment a second such tool (shape) existed. Replaced with a per-tool-id ref map; StylePicker resolution is now registry-driven off each tool's own descriptor field, not a hardcoded branch.
• Shape's armed state/style store/placement door/drag hook are instantiated inside useAtlasDragTools.ts (Eraser/Laser's own precedent, not Area/Pencil's) to keep AtlasBoard.tsx under the 500-line cap.

Measurement against the 26 → 21 → 16 curve

20 files touched, +909/-68 lines, one PR. Roughly in line with the falling per-tool marginal cost the registry bet predicted (S2:26, S3:21, S4: ~8/tool for two tools). commands.ts and useKeymapDispatch needed zero changes (as with every slice since S2); AtlasCreationTray/useAtlasCreation/useAtlasDragTools took small additive edits plus one real structural fix the second drag-to-draw tool exposed.

Findings reported, not silently worked around

1. A pre-existing, cosmetic bug: the .selected outline never visually renders on ANY board object (image/ink included, confirmed identical on an existing image object) — a clicked node also gains DOM focus, and React Flow's own .react-flow__node.selectable:focus { outline: none } wins at equal CSS specificity over Mill's .selected rule. Predates this slice (goal 0179 S1's own CSS), out of this slice's scope to fix. Selection state (multi-select, delete, undo) all work correctly — only the visual ring is invisible. Worth a follow-up.
2. Tray width: 538px (8 tools) → 604px (9 tools) — still narrower than the original six-button labeled tray (628px). The eraser/laser slice's structural pointer-events fix holds; atlas-page.spec.ts passes unmodified.
3. Caught my own e2e test bug via the goal-0184 class: a drag start point landing on the still-open style picker popover silently swallowed the gesture. Traced to root cause (not retried blind) and fixed by moving both drags into the board's clear top band.

Test plan

• atlasTools.test.ts extended: shapeTool.commit for rectangle/ellipse/arrow, registry shape/StylePicker assertions.
• New atlasShapeSvg.test.ts: pure geometry helpers (boxDimensions, arrowGeometry, shapePayload).
• New frontend/e2e/atlas-shape-tool.spec.ts (shared pool): draw+delete, type switching, style-persistence-across-rearm + Promote to card. All green against the real built server, not mocked.
• Regression sweep green: atlas-pencil-tool, atlas-eraser-laser, atlas-image-tool, atlas-page (14/14).
• Full npm run test (92 files / 779 tests), npm run lint, npx tsc --noEmit, npm run boundaries, go build/go vet, and the full lefthook run pre-commit suite all green.
• Screenshot taken (server mode + Playwright, deviceScaleFactor 2): image + a red-stroked, thick rectangle + an ink stroke drawn across it, shape selected (DOM-confirmed .selected, visual ring not visible per finding build(deps): Bump golang.org/x/oauth2 from 0.35.0 to 0.36.0 in the go-dependencies group #1 above).

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https://claude.ai/code/session_01FW5GkkAG8du7tNdYLk2zSd