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/