Harden synchronization and membership safety

[bitbeamer]

Addresses the blocking and high-severity findings from the 2026-08-22 solution review.

• preserve concurrent inbox edits and reject unresolved merge commits
• enforce writer/receive exclusion with a cross-process reader/writer guard
• commit during FUSE flush so publication failures reach applications, while quarantining payloads
• enforce administrator-rooted admission, role, invitation, revocation, and pin authorization
• validate proposed control-ref objects and signatures before managed receive updates refs
• permit authorized membership generations while rejecting deletion, rollback, and self-escalation
• bound managed request headers and restrict mesh discovery to accepted peers
• serialize recovery and remove scheduler starvation/shutdown hazards
• avoid holding the global write-session lock during content staging

Verification:

• go clean -testcache && make test
• targeted go test -race across repository, mount, membership, managed, and peer
• go vet ./...
• git diff --check