perf(useMouse): single-pass hit-test and pre-scale mouse coords

[chiefcll]

Summary

Cuts work on the mousemove hot path in useMouse without changing observable behavior.

• getChildrenByPosition: collapsed BFS filter → max-z scan → queue reassignment per level into a single fused loop. Removes the per-level currentLevelNodes allocation and the unreachable isTextNode branch.
• Pre-scale once: divide clientX/clientY by deviceLogicalPixelRatio once at entry, instead of multiplying every node's absX/absY/width/height on every collision test. Applied in both getChildrenByPosition and findElementByActiveElement.
• No throwaway arrays: .filter() + last-element pick replaced with a single reverse iteration in findElementWithCustomState and the focus effect.
• Hoist Config.focusStateKey out of the per-tick filter predicate.
• Dead code removed: findHighestZIndexNode (folded into the descent), unreachable if (!activeElm) return after the forwardStates walk-up, dead active && guard inside findElementByActiveElement's parent loop, unused isTextNode import.

For a level with N children, hit-testing drops from ~3N iterations + 1 allocation to N iterations and no allocations, plus 4 fewer multiplications per node tested.

Tests

New tests/useMouse.test.tsx (10 cases) covering:

• onMouseClick invoked on focused element
• Click ancestor walk-up via findElementByActiveElement
• Mousemove focuses the deepest matching child
• Z-index decides between overlapping siblings
• skipFocus and alpha: 0 exclude from hit testing
• hoverState applied on mousemove
• Hover transfers when moving between elements
• forwardStates propagates hover to ancestor
• deviceLogicalPixelRatio scaling
• mousedown applies pressedState, click clears it

All 10 pass against the previous main implementation and against this refactor — proves behavioral equivalence.

Test plan

• npm test — 120/120 (110 existing + 10 new)
• npm run tsc — clean
• Tests pass against pre-refactor useMouse.ts (verified via git stash)
• Tests pass against refactored useMouse.ts

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