Add test coverage retrofit for Phase 1/2 (worker, api, frontend)

Requested after Phase 2: from here on, MCMapper development follows
TDD (test-first) — this retrofits the pieces already built before that
request landed.

worker: unit tests for the tile rasterizer (background fill, exact
upscaled-block boundaries, full-grid painting, out-of-bounds columns) and
the block-color palette (distinctness checks, including that the
"unmapped block" placeholder never accidentally collides with a real
block's color). 16 tests total alongside the existing mesher tests.

api: wired up `bun test`. Unit tests for chunkOf's coordinate math.
Integration tests (real Postgres/Redis/MinIO, see README's new "Running
tests" section) for wsGateway.message() — auth accept/reject, upsert +
dedup on columns/sections, not-authenticated/invalid-JSON handling — and
for the tile/mesh/servers HTTP routes, driven through Elysia's in-process
`.handle()` rather than a bound port (sidesteps the stale dev-server
port-collision issue hit repeatedly this session). index.ts now exports
`app` and only calls `.listen()` when run directly, specifically so tests
can drive it this way.

frontend: extracted mesh.js's binary-format parser into its own ESM
module (mesh-format.js) so it's unit-testable without a browser/Babylon;
mesh.js now imports it. Tests build a buffer independently of the parser
(mirroring worker's encoder layout) so a mismatch in either direction —
Rust producer or JS consumer drifting — would be caught.

frontend/src/index.ts

@@ -14,6 +14,7 @@ const app = new Elysia()
   .get("/css/tailwind.css", () => Bun.file(join(import.meta.dir, "public/css/tailwind.css")))
   .get("/js/map.js", () => Bun.file(join(import.meta.dir, "public/js/map.js")))
   .get("/js/mesh.js", () => Bun.file(join(import.meta.dir, "public/js/mesh.js")))
+  .get("/js/mesh-format.js", () => Bun.file(join(import.meta.dir, "public/js/mesh-format.js")))
   .listen(Number(process.env.PORT ?? 3001));
 
 console.log(`[frontend] listening on :${app.server?.port}`);