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.

api/package.json

@@ -7,7 +7,8 @@
     "dev": "bun run --watch src/index.ts",
     "start": "bun run src/index.ts",
     "migrate": "bun run src/db/migrate.ts",
-    "seed": "bun run scripts/seed-server.ts"
+    "seed": "bun run scripts/seed-server.ts",
+    "test": "bun test"
   },
   "dependencies": {
     "cookie": "^2.0.1",