chore(mutants): src/diff/** audit — 0 surviving (#35, module 1/6)

docs/src/SUMMARY.md

@@ -48,5 +48,6 @@
 
 # Project
 
+- [Mutation testing audit](./internals/mutation-testing.md)
 - [Roadmap](./roadmap.md)
 

docs/src/internals/mutation-testing.md

@@ -0,0 +1,32 @@
+# Mutation testing (cargo-mutants)
+
+bomdrift's risk-bearing modules are audited with [`cargo-mutants`](https://github.com/sourcefrog/cargo-mutants) to verify the test suite actually catches the mutations the tests claim to catch. The audit is run manually before each release — it is **not** wired into CI (the runtime is prohibitive for per-PR feedback, and the signal is most useful at release-cut time).
+
+## How to reproduce
+
+Run from a clean worktree (the tool rewrites source files in place during the run):
+
+```sh
+git worktree add ../bomdrift-mutants main
+cd ../bomdrift-mutants
+cargo mutants --in-place --no-shuffle --file '<path-glob>' --timeout 60
+```
+
+Use a separate worktree so concurrent editing in your main checkout doesn't collide with the in-place rewrites.
+
+## Audit log
+
+Each entry: `module — caught / unviable / surviving / total — date`. The target is **<10% surviving**.
+
+### `src/diff/**` — v0.9.9 baseline
+
+- **Total mutants:** 19 (+ 1 baseline build)
+- **Caught:** 16
+- **Unviable:** 3 (compile-failures the tool excludes from the surviving-rate denominator)
+- **Surviving:** 0
+- **Surviving rate:** 0 / 19 = **0.0%**
+- Date: 2026-06-01
+
+No test additions required. The diff engine's existing unit and property tests fully cover the mutation surface.
+
+The three unviable mutants were mutations that produced code that does not compile — they're excluded from the surviving denominator because the compiler itself rejects them before the test suite runs.