feat(hex): zoom-reactive color legend for hex layers (#316)

[cboettig]

Closes #316.

Problem

Hex tile layers rendered no legend. Two independent blockers:

1. They bypass registerLayer (which wires the legend via _showLegendIfVisible), so the legend was never even invoked for them.
2. Even if invoked, deriveContinuousLegend only parses a top-level interpolate/step; hex paint is ['case', …, ['match', ['get','res'], …]], so it returns null.

Change — a hex-specific legend path (not a bent continuous deriver)

• addHexTileLayer stores valueStats + palette on the layer state, sets legendType: 'hex', and calls _showLegendIfVisible() (mirroring what registerLayer does for curated layers).
• _hexLegend(state) derives {colors, range, res} from the fixed 3-stop PALETTES (imported from hex-layer-helpers.js, the same source the map ramp uses) and the value domain of the resolution currently on screen.
• _currentHexRes(state) determines that resolution by sampling the rendered features' res property (the server pyramid serves one resolution per tile keyed off zoom, so this is more honest than reproducing the server's zoom→res formula client-side), with a finest-resolution fallback for the moment right after add / for GeoJSON layers whose features carry no res.
• _showLegend gets a hex branch: colorbar + min/max labels + an H3 resolution N caption.
• Zoom-reactive: a single lazily-wired moveend handler (_updateHexLegends) relabels every visible hex legend as the on-screen resolution changes. This matters because the per-res domain scales ~7×/step, so a static range would mismatch the colors at most zoom levels.
• removeHexTileLayer tears down the legend entry.

Design notes

• Reuses the exported PALETTES rather than re-parsing the case/match paint — the gradient is [c0,c1,c2] (min→max), matching buildFillColorExpression.
• Values format as grouped integers / 3-significant-digit floats (_fmtLegendValue).

Testing

• New test/map-manager.hex-legend.test.js (jsdom, 9 tests): _hasLegend for hex; _currentHexRes dominant-res sampling + finest fallback; _hexLegend palette+domain; _showLegend DOM (colorbar/labels/res note); _updateHexLegends relabel on zoom; removal cleanup; value formatting.
• Full suite green (525 passed).
• map-manager.js is browser-bound; visual check on a deployed app recommended (colorbar + label updates while zooming a hex layer).

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	58.91%	5400 / 9165
🔵	Statements	58.89%	5400 / 9169
🔵	Functions	65.54%	194 / 296
🔵	Branches	83.22%	1206 / 1449

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
app/map-manager.js	54.87%	82.52%	46.15%	54.93%	57-148, 155-159, 165-392, 401-445, 459-481, 522-558, 568-581, 589-602, 666-667, 684-685, 842-851, 879-880, 886-889, 898-908, 914-921, 977-980, 988-992, 995-1011, 1017-1018, 1024-1027, 1096-1097, 1117-1119, 1141-1146, 1152-1155, 1165-1183, 1190-1194, 1202-1295, 1302-1342, 1366-1368, 1380-1394, 1440-1445, 1457-1503, 1551-1552, 1619-1638, 1641-1656, 1666-1668, 1768-1775, 1837-1838, 1871-1875, 428-462

Generated in workflow #175 for commit 615d709 by the Vitest Coverage Report Action