Distribute — even out gaps in a multi-selection

[lklyne]

Closes #180

Adds a Distribute action (ADR 0015 D7) that evens edge-to-edge gaps in a loose 3+ entity selection along the dominant axis, keeping the first and last items fixed. The output is reorder-eligible by construction — distribute is the on-ramp to a reorderable row.

What's in this PR

Phase A — Pure kernel

• src/shared/reorder-row.ts: extract and export dominantAxis so the distribute kernel and reorder detector share one axis definition (no copy-pasted predicate)
• src/shared/distribute-row.ts: pure distributeRowPositions kernel — sort by leading edge, compute even gap, pack middle items, return only changed positions; null when <3 items or already even
• tests/unit/distribute-row.test.ts: 11 unit tests including the round-trip that proves output passes detectReorderableRow

Phase B — Commit path + CLI + HTTP

• src/main/runtime/document-commands.ts: distributeSelection(entityIds) mutator — reads live geometry, calls distributeRowPositions, writes changed positions via writeReorderedPosition inside one beginBatch/endBatch + markUndoBoundary. Position-only, no entityOrder, no managedLayout.
• src/main/routes/workspace.ts: POST /selection/distribute (defaults to current selection)
• src/main/cli-commands.ts: specular distribute <ids>
• src/main/routes/test.ts: POST /test/canvas-distribute-selection/commit
• tests/smoke/app-client.ts: distributeSelection helper
• tests/smoke/distribute-selection.test.ts: smoke tests — one transaction, one undo step, endpoints fixed, gaps equal, output reorder-eligible, no-op for already-even and <3 items

Phase C — Selection popup

• src/shared/types.ts: distributeSelection() on CanvasBgElectronAPI
• src/preload/canvas-bg.ts + src/main/ipc/register-canvas-entity-ipc.ts: IPC wiring
• src/renderer/above-view/DistributePopup.tsx: popup button (AlignJustify icon) shown for 3+ loose entities
• src/renderer/above-view/App.tsx: mount DistributePopup when ≥3 entities selected with no managed-row group

Phase C acceptance (in-app visual) requires human /verify: with 3+ unevenly-spaced items selected the Distribute button appears; clicking evens the gaps; reorder dots then appear on the result; one undo restores.

https://claude.ai/code/session_01QdaCmAuAZfcKYFGeesiNBP

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