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.

.gitignore

@@ -7,6 +7,9 @@ packages/bin/kitchen
 result
 result-*
 
+# test coverage output
+coverage/
+
 # direnv / nix-direnv cache
 .direnv/
 

bunfig.toml

@@ -0,0 +1,8 @@
+[test]
+# Coverage is opt-in via `bun test --coverage` (see the test:coverage script).
+# These settings only take effect when that flag is passed.
+coverageReporter = ["text", "lcov"]
+coverageDir = "coverage"
+coverageSkipTestFiles = true
+# test-support.ts is test scaffolding, not product code; keep it out of the numbers.
+coveragePathIgnorePatterns = ["**/test-support.ts"]

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"