build: set up test coverage via bun (task 0003)

Add a bunfig.toml enabling coverage with text + lcov reporters, a
test:coverage script, and gitignore the coverage/ output. Test files and
the test-support helper are excluded so the numbers reflect product code
only.

package.json

@@ -8,6 +8,7 @@
   ],
   "scripts": {
     "test": "bun test",
+    "test:coverage": "bun test --coverage",
     "lint": "biome check .",
     "format": "biome check --write .",
     "postinstall": "bun2nix -o bun.nix"