feat: add useMousePosition utility

[hugentobler]

Add useMousePosition — a reactive utility that tracks cursor position in viewport coordinates and optionally relative to a target element. Follows the onClickOutside pattern with start()/stop()/enabled listener control.

PR Checklist

• Code is formatted and linted (pnpm lint, pnpm format)

  • pnpm format — all files unchanged (clean)
  • pnpm lint — 0 errors, 0 warnings

• Types pass (pnpm check)

  • svelte-check — 0 errors, 0 warnings across both packages

• Tests pass locally (pnpm test)

  • 13 unit tests passing across 3 test groups:
    • Global position tracking (3 tests)
    • Element-relative position with negative/overflow values (6 tests)
    • Start/stop listener control with immediate option (4 tests)

• Docs updated (content + demo)

  • Docs page with Basic Usage, Advanced Usage (out-of-bounds tracking, controlled listener), Options, and Type Definitions
  • Interactive demo with element label, coordinate display, inside status, enabled status, and start/stop buttons

Files Added

• packages/runed/src/lib/utilities/use-mouse-position/use-mouse-position.svelte.ts — implementation
• packages/runed/src/lib/utilities/use-mouse-position/index.ts — barrel export
• packages/runed/src/lib/utilities/use-mouse-position/use-mouse-position.test.svelte.ts — unit tests
• sites/docs/src/content/utilities/use-mouse-position.md — docs page
• sites/docs/src/lib/components/demos/use-mouse-position.svelte — demo component

Files Modified

• packages/runed/src/lib/utilities/index.ts — added use-mouse-position export

🤖 Generated with Claude Code

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Latest commit: 6b4194e

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[hugentobler]

Thanks for reviewing. Added a new utility for my own usecase inspired by similar React hooks. Has an 'advanced' out-of-bounds usecase that composes nicely with other runed utilities.