07 — Caching, incremental analysis & the CLI gates in CI

[rahlk]

Part of #9. Phase 1 — level 1.

Learning goals

Integration testing a binary (TRPL Writing Automated Tests + the assert_cmd/predicates
crates), filesystem hygiene (tempfile), serde round-trips as a cache format, GitHub Actions for
Rust (dtolnay/rust-toolchain, Swatinem/rust-cache — already in your release.yml, reuse them).

Task

• Per-run output cache: write analysis_cache.json under -c after every successful run;
  on a lazy re-run, reuse per-file RustModules whose hash/mtime/size are unchanged and rebuild
  only what changed. --eager forces a clean rebuild and rewrites the cache.
• Flag validation everywhere: --format msgpack (until implemented) and any unknown --emit
  exit non-zero with a clear message — asserted by an integration test, not a promise.
• stdout contract: no -o → compact JSON on stdout, logs on stderr only (pipe-clean at -vv).
• CI: a test.yml running fmt + clippy -D warnings + the full test suite on push/PR, so every
  later issue's gate is un-regressable.

Teacher's notes

• The four caching behaviors to assert (from the skill's testing reference): CacheFileWritten,
  CacheContentsRoundTrip, SecondRunReuses, EagerForcesRebuild (sleep ≥1s so mtime can advance).
• assert_cmd drives the real binary — this is where the stdout/stderr split gets honest.

Gate

• All four caching tests + both flag-validation tests green, in CI.
• A -vv run piped to jq . parses — logs never contaminate stdout.