Foundation for the cost estimates and partial-stall detection that
Phase 2 will surface. No consumers yet — this PR just lays the table,
the seed rows, and the lookup helper so subsequent UI work has
estimates available without a per-ZIM benchmark.
## What lands
- New table `kb_ratio_registry` (pattern, chunks_per_mb, sample_count,
notes). Migration creates and seeds heuristic defaults from the RFC
appendix: devdocs (1100/MB), Wikipedia variants (270/MB), iFixit
(50/MB), Stack Exchange Q&A (200/MB), video-only ZIMs (0), plus a
catch-all fallback at 100/MB.
- `KbRatioRegistry` model with static `lookup()` and `estimateChunks()`.
- Pure helper `kb_ratio_lookup.ts` doing longest-prefix-match — a
specific entry (`wikipedia_en_simple_`) overrides a broader one
(`wikipedia_en_`). 9 unit tests covering the lookup boundary.
- `sample_count` starts at 0 (heuristic seed) and is reserved for
Phase 4 self-calibration to increment as observed ZIMs update each row.
## Not in scope
- Self-calibration on successful ingestion (Phase 4)
- UI consumers — Warning B (partial-embed stall) and the storage budget
meter / time estimates land in Phase 2.
## Tested
- Type-check clean
- 9 unit tests pass for `findChunksPerMb` and `estimateChunkCount`
- Migration applied on NOMAD3 via hot-patch; 9 seed rows verified in DB
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