feat: add divergence.bars diverging bar chart island

[lukaisailovic]

What

Adds divergence.bars — a diverging bar chart. One signed bar per category extends up or down from a zero baseline, each bar colored by which value band it falls into. It fills the gap between category.bar (compares unsigned groups) and waterfall.bars (accumulates into a running total): independent signed bars where the up/down direction is the message — calorie surplus/deficit, budget variance, net cash flow, sentiment, temperature anomaly.

Interface

Happy path is one line; everything else is optional with sensible defaults:

{ "type": "divergence.bars", "dataset": "calorie_delta", "x": "date", "value": "delta" }

• buckets: [{ gte?, lt?, color, label? }] colors each bar by the band its value falls in (matched half-open [gte, lt); labels drive the legend). Omit for a green-positive / red-negative two-tone.
• No baseline / colors / transform knobs by design — compute the signed value in SQL so the manifest stays declarative.

Changes

• schema: DivergenceBars + every registration (both unions, the three span maps) and the islandRequirements binding check
• runtime: lazy DivergenceBars renderer + registry; pure bucketColor / buildDivergenceBars (unit-tested)
• cli: islandSkeleton entry
• docs: islands catalog + charts page + a concepts note on filter bind; regenerated manifest reference
• examples/health: a "Daily calorie delta" chart on the Nutrition page (via a calorie_delta SQL transform), wired into the page's period filter
• skill: adding-new-islands — the end-to-end island-add discipline (interface/API design → .describe() copy → file-by-file wiring), in .agents/skills with the matching .claude symlink

Testing

pnpm build && typecheck && test && lint && validate:templates green locally; new unit tests cover bucket boundaries and null handling. Verified live in the examples health dashboard.