fix(#111): cap chart X categories, not raw rows; honest transform note

[BorisTyshkevich]

What & why

Fixes #111 (follow-up to #109). buildChartData() applied the chart-type
row cap to the raw rows before grouping, so the chart could disagree with
the result already in memory: a category was short-totaled when another row for
the same cell appeared after the cap, and a category could vanish only because
its first row appeared after the cap. The old note also said first N of M rows
while the chart displays categories, and — because the note lived inside the
controls block — Dashboard and read-only/detached tiles (controls:false)
capped categories silently.

Fix

• Cap X categories, not raw rows. Two-pass buildChartData(): pass 1
  discovers unique raw X keys across all fetched rows in first-seen order and
  retains the first chartRowCap(type); pass 2 aggregates every fetched row
  whose X is a retained category. No value-ranking, sorting, or Other bucket.
• Typed metadata. ChartDataResult.meta: ChartDataMeta
  (totalRows/totalCategories/shownCategories/categoriesTruncated/
  duplicateCellsSummed/groupKey).
• Honest note. New pure chartDataNote(meta) →
  first N of M categories and/or duplicate X[/series] rows summed in the browser. It describes only the browser-side transform, never whether the SQL
  pre-aggregated. Duplicate detection keys on X (or (X, series)), so a normal
  multi-series result with repeated X values is not reported as summing, and
  a duplicate confined to an omitted category never flags the visible chart.
• Every surface discloses. Note renders inline in the config bar on the
  workbench, and as a standalone block above the canvas on controls:false
  surfaces (Dashboard / read-only / detached).
• Aggregated once per render — chartJsConfig accepts the precomputed
  ChartDataResult via a new data option.

No panels.ts change was needed (the note now renders regardless of controls);
no new runtime dependency and no new network request.

Tests

• chart-data.test.ts: late-row contribution, first-seen order, post-cap
  omission, all metadata states (truncation-only, duplicate-only, both,
  unique multi-series, duplicate (X, series), duplicate in an omitted
  category, series only in omitted categories), chartDataNote formatter, and
  chartJsConfig honoring precomputed data.
• chart-render.test.ts: category-cap note text, type-switch recompute,
  duplicate-X / duplicate-X-series / combined text, standalone note on
  controls:false, no-note when neither condition holds, and single
  aggregation per render.
• panels.test.ts: a Dashboard chart panel discloses the cap despite hidden
  controls.

npm test (2891 tests, per-file 100/100/100/100 gate) and npm run build pass.

Closes #111.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency (or it's a deliberate, justified addition — see CONTRIBUTING)
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it

🤖 Generated with Claude Code

https://claude.ai/code/session_01XP1KN94N969SVMNFPdVtGg