Backport the pinned Arduino 2.0.17 HWCDC suffix and interrupt handling into a build-local source copy. Preserve unsent bytes, serialize ISR and producer rearm, and include FIFO, suffix and active writers in transport cleanup proofs without changing the loop stack allocation.
Validate zero and short writes, producer races, quarantine and stale cleanup with sanitizer-backed native tests and negative controls. Keep other frameworks and USB modes on their existing drivers; update the watchdog file mock for the existing buffer API.