Make sync state ownership process-neutral

[myobie]

What changed

• Give SyncEngine explicit config and state paths instead of a FabricHome.
• Add explicit-path load and save APIs for SyncBook.
• Replace the daemon validation-log dependency with the sync-owned fabric::sync target.
• Keep peer addresses and policy inside the daemon transport; the engine receives an opaque key and display name.
• Hold <state-root>/owner.lock for the complete engine lifetime.
• Keep the embedded daemon engine as the production owner.

Red proof

Before the production change, the new owner property reached and parsed deliberately corrupt state in a second engine. It failed because no owner lease existed.

After the change, the second engine fails on the owner lease before it reads state. Dropping the first engine releases the lease and permits a restart.

Verification

• cargo check --all-targets — passed
• cargo test --lib — 471 passed, 4 ignored
• engine suite — 103 passed, 3 ignored
• real Iroh sync and restart — 3 passed in 136.30 seconds
• folder convergence matrix — 18 passed in 223.80 seconds
• unchanged five-second pipe property — 502 samples, 10.005 ms source maximum, 24.718 ms delivery maximum, 8 scans
• git diff --check — passed

This pull request changes no durable format and activates no companion path. Reverting it removes only the boundary and owner lease. No release or deployment is included.