Replace the 'known remaining inefficiency' section with the layout that now exists, plus the measured before/after (clean release build: 2m14s -> 1m46s wall, keydr units 61.0s -> 32.4s, peak ~2.0 GB unchanged). Add a maintenance note: new modules go in src/lib.rs, not src/main.rs. Re-declaring them in the binary would silently restore the double compile this refactor removed.