feat(sliders): support interpolation on merged/banner header tables

[IanMayo]

Summary

Speed-vs-length matrix tables commonly draw a merged banner above the numeric column headers. Two author permutations are now supported:

1. Full-width banner — a single <th colspan="N">Length Overall (m)</th> row above the Speed Knots | 10 | 20 | … header row.
2. Rowspan corner — a <th rowspan="2">Speed Knots</th> corner beside a <th colspan="N"> banner, with the numeric length headers on the leaf row below.

Previously the canonical addressing layer treated the first header row as the column header. The banner (or the collapsed column count it produced) hid the numeric axis, so the table read as not suitable and the interpolation (slider) enrichment was never offered — exactly the "extra header cell will probably prevent that" case reported.

What changed

Header resolution now peels off leading banner/grouping rows and returns the leaf column-header row that aligns 1:1 with the data columns. A new occupancy-aware (colspan + rowspan) column reader handles the rowspan corner, which shifts the leaf cells right. The fix threads through every consumer of the header model:

• core/table-grid.ts — resolveHeader (banner peel) + headerRows, sourceHeaderColumns, dataHeaderCells, sourceColumnMatrix.
• core/type-detection.ts / ui/toggle-injector.ts — flatten a multi-row header by logical column so suitability + column typing see the real numeric columns instead of a collapsed banner.
• ui/header-utils.ts — place per-column lozenges on the leaf headers, the corner slider lozenge on the true corner, and row lozenges on the body rows.
• enrichments/slider-injection.ts — read axis headers via dataHeaderCells; inject the row-slider header cell into the top header row spanning the whole block so the gutter lines up with the body.

Every banner-aware branch is gated on headerRows(table).length > 1, so single-row-header tables are byte-identical — no behaviour change for the common case.

Testing

• src/core/__tests__/table-grid.test.ts — leaf-row resolution + dataHeaderCells for both permutations, no-<thead> banner, and a plain single-row equivalence check.
• src/enrichments/__tests__/slider.test.ts — buildAxisBinding for both permutations, end-to-end dual-axis slider creation + interpolation on a rowspan-corner table, and clean injection teardown.
• src/ui/__tests__/toggle-injector.banner-header.test.ts — drives the real activation path and asserts the enabled slider lozenge appears for both permutations, plus byte-identical activate→deactivate teardown.

Full jsdom unit suite: 731 passing (94 files). tsc --noEmit clean; vite build clean. The Storybook browser project + Playwright e2e couldn't run in this environment (Playwright browser-build version mismatch), so those weren't exercised here.

Notes / follow-ups

• The lozenge-mount fix places the corner slider affordance correctly; broader per-column sort/filter/heatmap behaviour on multi-row-header tables is offered but not exhaustively re-validated for exotic combinations (e.g. banner header + virtual columns, which fall back to source-only column typing).
• A demo page for the merged-header pattern could be added under demo/ if useful for visual verification.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EvweKwJZVqe5rWL8bF6ogS

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Generated by Claude Code

[github-actions]

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[codacy-production]

Up to standards ✅

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🟢 Metrics 53 complexity · 2 duplication

Metric	Results
Complexity	53
Duplication	2

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