feat(search): local POI lane (PR 3 of 5)

[taddyb]

Summary

Third PR in the rollout. Wires the 79k-POI binary shipped by PR 2 into the search bar as a synchronous-ish "local" lane that returns results before any network round-trip.

What lights up

• Typing "Battery Park" with no bias now resolves to the actual park (local exact-name hit, score 100 + park category boost 8), not whatever Photon happened to rank first
• Typing "Joe's Pizza" returns local Joe's first across the merged dropdown — local exact-name beats Photon's noisy ranking
• Address queries (e.g. 140 W 25th St) still route to Census + Photon as in PR 1 — the local lane returns empty for non-POI queries and the dropdown falls back to the network lanes

How it works

`js/poi-index.js` loads `tiles/pois.bin` (or assembles from chunks) on the first keystroke, decodes the POI1 binary, and builds two in-memory indexes:

Index	Shape	Purpose
Postings	`Map<token, Uint32Array>`	Token search — query tokens intersected with prefix-expanded last token
ByCategory	`Map<category, Uint32Array>`	Future use by PR 5's discovery lane

`scoreLocal` ranks each candidate:

• 100 if normalized name === query exactly
• 80 if every query token is in the name
• 60 if just the last token is a prefix of any name token
• • 15/8/5 for transit/park/attraction categories
• − `log2(name_token_count)` length penalty

`searchMerge` adds the local lane and gains an `overrides` parameter (`{local, photon, census}`) so tests can inject stubs without touching globals.

Test plan

• `node --test tests/unit/**/*.test.mjs` → 30/30 pass
• Manually: "Battery Park" returns the park as the top dropdown item, no network needed
• Manually: "Joe's Pizza" returns Manhattan / Brooklyn Joe's above Photon noise
• Manually: "140 W 25th St" still resolves via Census (PR 1 path not regressed)
• Manually: with browser DevTools, observe that the network panel shows zero requests when typing a landmark name (local lane fires first)

Subsequent PRs

• PR 4: WASM `reachable_nodes` method
• PR 5: search-driven category discovery + map pins

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