Reduce chart hover layout and sorting work

[ktsaou]

What changed

This PR removes four measured hot-path amplifiers without changing chart queries, payloads, refresh frequency, rendered values, or synchronized-hover behavior:

1. Reuse Intl.Collator instances during dimension sorting.
2. Coalesce synchronized hover broadcasts to one update per animation frame and skip identical positions.
3. Replace the value font-sizer's linear write/read loop with a bounded binary search.
4. Cache popover geometry and move it with a frame-coalesced transform.

Each optimization is isolated in its own commit.

Why

A paused-dashboard hover profile showed 30 pointer movements consuming:

• 2,983 ms of main-thread task time
• 665 ms of JavaScript
• 271 ms of layout
• 103 ms of style recalculation
• 1,541 layouts and 1,642 style recalculations

A separate 20-movement audit recorded 3,399 DOM mutations, including 911 mutations on latest-value labels. The dominant problem was synchronized update fan-out plus repeated geometry and font-fit measurements.

The dimension-sort microbenchmark also showed that reusing the collator reduced locale-aware sorting of 6,000 tied dimensions from 19.665 ms to 0.504 ms per sort, and grouped ties from 46.909 ms to 1.358 ms per sort.

The font-fit path now needs at most seven layout probes instead of as many as forty. The popover tests verify that subsequent pointer movements perform no geometry reads.

Compatibility

• Synchronized charts still share the timestamp while resolving their own rows.
• The latest pointer event wins within each frame.
• Pending hover work is cancelled on blur and teardown.
• Popover alignment and edge flipping are preserved.
• No artificial refresh backoff or global scheduling policy is introduced.

Validation

• Full Jest suite: 156 suites passed; 1,534 tests passed and 2 skipped.
• Production build: 488 CommonJS and 488 ES module files compiled.
• ESLint passes for every changed file.
• Isolated Storybook check: synchronized charts rendered, hover followed the latest pointer position, viewport-edge flipping worked, and no console errors occurred.