feat: snap-to-grid for entity drag

[lucasfelixs]

Summary

• Adds a snapToGrid(value, gridSize) pure utility to @floormap-tools/core that rounds a world-space coordinate to the nearest grid multiple
• Wires the option through both handler implementations (packages/svg/src/handlers.ts and packages/adapters/react/src/internal/attachHandlers.ts) and exposes it as snapToGrid?: number on SvgRendererOptions and FloormapCanvasProps
• Snap is applied to the entity's absolute position (not the drag delta) on every pointermove, preventing floating-point accumulation across frames
• Adds UI controls to both example apps — a checkbox to toggle snap and a number input for grid size; the vanilla JS example remounts the renderer on change since mountSvgRenderer bakes options at mount time

Changes

Package	What changed
@floormap-tools/core	New snapToGrid export in utils.ts; 5 new unit tests
@floormap-tools/svg	snapToGrid?: number option on SvgRendererOptions; applied in handlers.ts; 3 new integration tests
@floormap-tools/react	snapToGrid?: number prop on FloormapCanvas; wired through attachHandlers
examples/js-vanilla	Snap toggle + grid-size input; destroy/remount pattern
examples/react	Snap toggle + conditional grid-size input via React state
.claude/skills	New /issue skill for creating and editing GitHub issues

Test plan

• pnpm test — all 159+ tests pass, including new snapToGrid unit tests and handler integration tests
• pnpm typecheck — no errors across all packages
• Open http://localhost:5173 (vanilla) or http://localhost:5174 (React), enable "Snap to grid", drag an entity — position should snap to the nearest 40-unit grid point
• Change grid size to 20, drag again — tighter snap grid
• Disable snap, drag — free movement, no snapping

Closes #40

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