feat: add scrub.snapToCandles — crosshair snaps to candle centers

[ianlapham]

What

• New opt-in ScrubConfig.snapToCandles (default false): quantizes the scrub X to the hovered candle's center, so the crosshair — and everything derived from it (time, tooltip, trailing dim) — jumps tick-to-tick, TradingView-style, instead of gliding.
• Gaps between candles and line mode pass the raw X through; the Candlestick demo gains a Snap to candles switch.

Why a pure helper in crosshairShared

• snapScrubXToCandleCenter (the inverse of computeScrubTime, then the forward mapping of the picked candle's center via pickCandleAtTime) is unit-testable under Jest; the gesture-side gate stays a thin istanbul-ignored worklet like its siblings.

Why LiveChart only

• LiveChartSeries has no candle mode, so the flag is wired through useCrosshair's two plain-scrub call sites (startPlainScrub / updatePlainScrub) and is inert everywhere else. Default behavior is unchanged.

Test plan

• resolveScrub default/carry cases; snap-math unit tests (center quantization across a bucket, live candle, gap + pre-history pass-through, zero-width plot/window guards); render coverage in candle and line mode with the flag on.
• npm run verify green; coverage thresholds hold (branches 91.6, functions 95.3, lines 97.0, statements 96.2).
• The gesture handlers are UI-thread worklets (istanbul-ignored like their siblings), so snap-while-dragging wasn't exercised under Jest; the equivalent behavior has been running in production at FOMO.

🤖 Generated with Claude Code

[brandtnewlabs]

Reviewed, fixed, and QA'd.

What I found:

• The snap-to-candles implementation itself looks sound; I found no functional blocker in the code.
• The public API is covered by typed JSDoc, the API reference, the scrubbing and candlestick guides, and the changelog.
• The example app exposes the feature on the Candlestick screen with a Snap to candles toggle.
• The merge blocker was a single CHANGELOG.md conflict with current main.

What I fixed:

• Merged the latest main into this branch.
• Resolved CHANGELOG.md by preserving both the candle-snapping Added entry and the existing marker/badge Fixed entries from main.
• Pushed the resolution in commit a77fca4.

Validation:

• npm run verify passed: typecheck, lint, and all 99 test suites (1,511 passed, 5 skipped).
• React Doctor: 96/100, no PR-specific issues.
• Agent-device QA passed on an iPhone 17 Pro simulator: nearby scrub positions stayed quantized to one candle center/time and then jumped discretely to the next candle; the restored app build logged no JavaScript/runtime errors.

The PR is mergeable now; CI is rerunning on the updated head.