docs: red-green rule for bug fixes; regenerate traceability matrix

CLAUDE.md now states the failing-test-first rule explicitly: write a test
that reproduces the bug and watch it fail before applying the fix, and
extract buried logic into a plain .ts module so it can be unit-tested. A
test written against already-fixed code can pass for the wrong reason.

CLAUDE.md

@@ -266,6 +266,23 @@ tagged responses keep the Rust field names as-is (e.g. `new_url`, not `newUrl`).
 
 ## Testing
 
+### 🔴 Bug fixes: failing test FIRST, then the fix
+
+When fixing a bug, **write a test that reproduces it and watch it fail before
+touching the fix.** Red → green, in that order:
+
+1. Write a test that exercises the broken behavior and **run it — it must fail**,
+   proving the test actually catches the bug (a test that passes before the fix
+   proves nothing).
+2. Apply the fix.
+3. Re-run — the test now passes, and so does the rest of the suite.
+
+Never fix first and backfill the test afterward: a test written against
+already-fixed code can pass for the wrong reason and silently fails to guard the
+regression. If the logic is buried in a component, extract the pure part into a
+plain `.ts` module (e.g. `episodeStrip.ts`) so it can be unit-tested — the same
+pattern as `TrackList.logic.test.ts`.
+
 ```bash
 # Rust
 cd src-tauri && cargo test