Freeze and test the dormant sync IPC bridge

[myobie]

Summary

• define the versioned fabric/sync-ipc/1 request, response, status, shutdown, and structured-error contract
• add a 16 KiB control-frame limit and five-second handshake bounds
• reject unknown required features while accepting additive optional fields
• bind mode-0600 sockets, verify the socket owner, verify client credentials, and validate a daemon-instance nonce
• switch to raw fabric/sync/1 bytes after a ready response
• implement the process-neutral SyncTransport client without selecting it in production
• keep the in-process reference server inside tests only

Red proof

The focused library target failed before implementation because the IPC listener, nonce, messages, relay, and transport did not exist. This established the complete missing bridge surface before production code was added.

Verification

• IPC bridge suite: 11 passed
• full library: 481 passed, 4 ignored
• binary: 19 passed
• cargo check --all-targets
• git diff --check
• permanent five-second sync/exec property: 502 samples, source max 10.002 ms, delivery max 13.264 ms, 9 scans; bound remains 150 ms

The conformance test resolves a peer and runs the real sync wire client through the Unix bridge to the test-only reference server. Separate tests cover both raw relay directions, half-close, owner-only permissions, nonce refusal, version refusal, required features, authenticated inbound identity, oversized frames, status, shutdown, and a stalled handshake.

No production request selects this path. The embedded engine remains the owner. No release or deployment is part of this change.