perf(core): move vec0 orphan discovery off startup and the event loop #1681

Problem

Embedding startup always runs full gcVec0DanglingRows(db()) in vec0 mode, even after migration and rechunk state have converged. Temporal orphan discovery uses a whole-table anti-join on the writer connection and blocks startup.

Evidence

• An equivalent temporal dangling-row anti-join took 2.87 seconds: 2.24 seconds user CPU and 0.42 seconds system CPU.
• Durable temporal migration state was already done=1 with no remaining rows, so the scan was unrelated to useful backfill work.

Required fix

Discover orphans in bounded pages on a read worker during idle maintenance. Return exact chunk IDs and delete them on the writer connection. Never rerun a timed-out scan synchronously.

Acceptance criteria

• Local embedding prewarm and startup never await a full vec0 scan.
• Orphan discovery is bounded and off-thread.
• Cleanup deletes exact chunk IDs only.
• Timeout or worker failure degrades safely and never blocks startup.
• Backed rows survive while dangling rows are eventually removed.