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.

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"]