Refactor: split ResultPanel.tsx (2,276 lines) along its natural seams; unify the duplicated pen-snapping pipelines

[dompm]

ResultPanel.tsx mixes pure geometry, three drawing tools, keyboard handling, toolbar config, and overlay UI in one 2,276-line component. Beyond readability, the structure is actively producing bugs — most notably:

The pen tool has two separately-implemented ~150-line snapping pipelines — hover preview (updateHoverPoint, :703-842) vs click commit (:1034-1190). The click path applies an extra "ABSOLUTE SHIFT CONSTRAINT ENFORCER" (:1139-1180) that the hover path lacks, so when Shift-drawing near a crop edge or fraction line, the marker shows one point and the committed vertex lands somewhere else. The duplication guarantees future drift.

Extraction plan, in order of payoff

1. utils/penSnapping.ts — lines 266-648 (getSquareSegmentDistance, simplifyPath, isStructuralCorner, findPenSnapTarget, getCanvasSnapping, findAlignmentGuides, findShiftAlignmentGuides, findLengthSnap) are pure functions with zero component coupling. Extract → unit-testable (nothing in the repo is tested today), ~400 lines gone.
2. usePenTool() + <PenOverlay> — the pen state cluster, updateHoverPoint, the pointerdown pen branch, and the render block (:1789-1930). Design it so the click commit consumes the already-computed hover point instead of re-running a second pipeline — fixes the WYSIWYG bug by construction.
3. useCanvasShortcuts(panel) — one shared hook replacing the near-duplicate keydown effects in ResultPanel (:922-990) and SheetPanel (:196-220), with modifier guards, panel scoping, and a single registration point (fixes the modifier/both-panels/ordering bugs filed separately).
4. <PolygonEditHandles> — vertex/midpoint drag handles (:2034-2159) + the draggedCorner/draggedMidpoint/dragStartPolygon state cluster.
5. <SolderPopover> (:1516-1626), toolbar tool definitions (:1404-1512, a static factory over t), onboarding cards (:1632-1666), lamp underlay (:1693-1749), and PieceOverlay to their own files. (DragHandle + getTooltipAnchor get deleted outright if Docked right-side inspector (replace floating piece-properties popover) #34 lands.)

Result: ResultPanel becomes a ~500-line orchestrator (viewport, tool dispatch, layer composition).

SheetPanel benefits from the same shortcuts hook and from de-duplicating its measure-tool init (the handleToolChange measure block at SheetPanel.tsx:377-403 vs the effect at :241-262 is exactly the duplication that let the forceTool ReferenceError rot unnoticed).