fix(serving): serialize registry writers across processes

Acquire the same stable sentinel before the broker and brain reaper load the
registry, retaining it through snapshot side effects and explicit publication.
Atomic replacement alone cannot prevent a stale writer from erasing an
acknowledged attachment or persisted name-allocation history.

Keep lock acquisition fail-closed, migrate live-daemon fixture writers, and
replace thread-only exclusion evidence with deterministic two-process storage
regressions, including abrupt-holder-exit release. Activate the hazard's doc,
impl, unit and int stages together. Both broker and brain must run the fix.

Fixes BigscreenVR/spt-bs-releases#308

Co-authored by: todlando