LEARNING.md indexes ten concept-to-file pointers (the spec floor was
five) — the newtype pattern, exhaustive enum match, sans-IO, trait
extension seams, thiserror + hot-path match-and-continue, Arc<Mutex>
vs Arc<RwLock>, DashMap, str0m's single-mutation invariant, graceful
shutdown, include_str!. fuzz/README.md pre-paves the layout (no
hostile-bytes surface in slice 1; harnesses land at step 5 per the
out-of-scope table). README's new dev-loop section documents the
libopus FFI prerequisite and the manual e2e steps.
