refactor(foundation): foundation.registry's _MISSING = object() bypasses the sentinel convention #2555

From the 2026-11-01 conventions audit, re-verified on 73add05a3. sentinel-convention.rst, "Naming a Sentinel": a package-authored sentinel is a named class instance housed in pcapkit.corekit.sentinels and listed in the page's table, whether or not it is public.

• pcapkit/foundation/registry/protocols.py module attribute _MISSING = object() ("Sentinel for a name pcapkit.protocols.__proto__ did not hold") is compared by identity in _NameLease.displaced, _release_name and _register_name (lease.displaced is _MISSING, protocol_registry.get(name, _MISSING)).
• git grep -n "= object()" -- pcapkit finds no other hit. It arrived in c3d4120 (fix(registry): unregistering a custom protocol removes its name from __proto__ (#1505) #1530), after object() sentinel sweep for #640: three sites left, each a judgement call rather than a mechanical pass #661 swept the others; it is not ported code, so the exemption does not apply.

Fix: add a <SENTINEL>Type class and a SCREAMING_SNAKE instance to pcapkit/corekit/sentinels.py, list it in the page's table, import it in protocols.py, and test identity and repr. No behaviour change.
Blocked: protocols.py is also edited by #2521, #2527 and #2544; unblocks when all three merge (gh pr view <n> --json state -q .state prints MERGED).