Layer editor: fill a tile layer from a VEDA STAC collection

[BhattaraiSijan]

Closes #333 (part of epic #331). Builds on the time-policy vocabulary from #332's PR (independent code; best merged after it so the written now policies render).

Summary

Adding a VEDA dataset meant hand-collecting facts a machine can read — tile endpoint, render parameters, temporal coverage, bbox, description — and typing them into the layer form. The tile layer editor's Actions row now has VEDA STAC Source: a two-step dialog that looks a collection up, shows what it offers, and fills the layer from the collection's own metadata.

• Look Up fetches collection-level metadata only (no item crawling) and presents the facts: title, the available renders (one distinct option reads as a plain fact line; several become a dropdown with identical duplicates collapsed; none disables filling), and the temporal coverage with granularity (2024-12-18 → ongoing · P1D).
• The admin makes the only two genuine choices — which render and time-enabled vs static — both preselected from the facts (dashboard render per VEDA convention; a span or open end suggests time-enabled).
• Fill Layer writes name, tile URL (titiler-pgstac collection mosaic; time-enabled adds datetime={starttime}/{endtime}, static omits the filter), a legend sampled from the render's colormap across its rescale range (legend and tiles agree by construction), bounding box, description, and the time block — where an ongoing collection (null extent end) gets dataEndTime: "now" plus interval/isPeriodic, so the layer stays current as data grows.

The system decides nothing silently: collections without renders are refused with a clear message (no asset guessing), band counts are never inferred, analysis support and layer identity (uuid) are untouched, and every filled field stays editable.

Commits

1. Engine — pure, I/O-free mapping (scripts/lib/vedaStacLayer.js), 21 specs on fixtures recorded from the live catalogs.
2. Backend — POST /api/stac/vedastac/inspect + /fill beside the existing STAC routes (admin-only, validated inputs, at most two upstream requests, warnings surfaced); fetch orchestration with injectable fetch, 6 fake-fetch specs.
3. Dialog — the two-step UI wired into the tile-layer metaconfig.

Testing

27 unit specs; full suite green. Verified live against both dev catalogs (screenshots in #333):

• Closed daily collection (sentinel2-nbr-daily): concrete dates, P1D, gray legend matching the tiles.
• Single-render (landsat-ndvi-daily) → fact line; multi-render (opera-displacement-daily, opera-dswx-daily, sentinel-1-coherence-yearly with P1Y) → dropdown with the VEDA default preselected.
• Ongoing collection (blackmarble-all-vars-daily-addMetadata): fills with end now — its timeline bar reaches the current date, while closed collections' bars correctly stop at their real ends.
• Filled layers render on the map with their generated URLs and legends.

Not yet exercised (follow-up verification planned): the no-renders refusal, no-temporal-extent collections, very large render lists, missing dashboard:time_interval, and the error paths (bad id, unreachable raster root) — all covered by unit specs, pending eyes-on checks.

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