test: add coverage reporting with regression thresholds

Add v8 coverage via a test:coverage script and vitest config: measure src
only, exclude the types-only deps.ts and the bin entrypoint main.ts, and
gate on ratcheted thresholds (statements 85, branches 78, functions 90,
lines 85) set just under current coverage. CI runs the unit+integration
tier through coverage so a real drop fails the build.

.gitignore

@@ -1,2 +1,3 @@
 node_modules/
 dist/
+coverage/