Add crosshair and shared cursor primitives

[tannerlinsley]

Summary

• add a renderer-native crosshair mark with x/y rules, labels, marker, clipping, ordering, facets, motion, and export support
• add shared focus and free cursor controllers with browser and React Native host bindings, synchronization, pinning, ownership, and accessibility behavior
• make point-free rule marks focusable through semantic anchors without adding pointer hit targets
• preserve SVG, Canvas, custom renderer/resource, React Native, and granular bundle boundaries
• migrate the axis-pointer, synchronized-cursor, free-cursor, and cursor-motion catalog cases to the new primitives

Why

Issue #32 exposed a real gap: whenFocused resolved candidates only from interaction points, while ruleX and ruleY intentionally emit no interaction points. Applications therefore had to build renderer-specific rectangle or SVG overlays to get axis pointers and crosshairs.

This keeps rule hit testing unchanged, adds non-interactive semantic focus anchors, and introduces proper crosshair/cursor primitives for both focus-snapped and free cursor use cases.

Validation

• pnpm test — 832 DOM tests and all six adapter/SSR targets
• pnpm package:check — packed exports, declarations, runtime consumers, Metro, Expo, and framework adapters
• pnpm react-native:poc:types
• pnpm canvas:check — browser-backed Canvas focus regression
• pnpm benchmark:check — 60-case cross-library comparison bundle baseline
• node scripts/measure-bundles.mjs --check
• pnpm exec tsc --noEmit --pretty false -p tsconfig.json
• pnpm exec prettier --check .
• pnpm docs:sync and documentation contract checks
• catalog contract, production build, loading budget, and browser regressions at 320px and 640px
• GitHub Chart library benchmarks workflow — all static, bundle, comparison, and stress partitions passed

Fixes #32

[coderabbitai]

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