Keep sync filesystem work off the runtime

[myobie]

What changed

• Add the permanent one-worker sync and exec pipe latency property test.
• Run scan, materialization, metadata, JSON, and persistence work on Tokio's bounded blocking pool.
• Snapshot node and disk state under short locks, then validate revisions before applying each result.
• Use copy-on-write node snapshots, so a clean phase clones no manifest paths or content bytes.
• Keep the existing per-entry operation guard, durable schema, wire format, and crash-write order.

Red proof

The test commit failed before the production change over its fixed five-second window.

• Samples: 502
• Source maximum: 10.415 ms
• Delivery maximum: 1.195 s
• Full scans: 11
• Delivery bound: 150 ms

Green proof

The unchanged test passed after the production change over five seconds.

• Samples: 502
• Source maximum: 10.003 ms
• Delivery maximum: 16.035 ms
• Full scans: 8

The following checks also passed locally:

• cargo check --all-targets
• cargo test --lib: 469 passed and 4 ignored
• cargo test --lib sync::engine::tests:: -- --test-threads=1: 102 passed and 3 ignored
• cargo test --test folder_sync -- --test-threads=1: 18 passed in 222.42 seconds

No release or deployment is part of this pull request.