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## Problem L0 compaction frequently gets starved out by other background tasks and image/GC compaction. L0 compaction must be responsive to keep read amplification under control. Touches #10694. Resolves #10689. ## Summary of changes Use a separate semaphore for the L0-only compaction pass. * Add a `CONCURRENT_L0_COMPACTION_TASKS` semaphore and `BackgroundLoopKind::L0Compaction`. * Add a setting `compaction_l0_semaphore` (default off via `compaction_l0_first`). * Use the L0 semaphore when doing an `OnlyL0Compaction` pass. * Use the background semaphore when doing a regular compaction pass (which includes an initial L0 pass). * While waiting for the background semaphore, yield for L0 compaction if triggered. * Add `CompactFlags::NoYield` to disable L0 yielding, and set it for the HTTP API route. * Remove the old `use_compaction_semaphore` setting and compaction-scoped semaphore. * Remove the warning when waiting for a semaphore; it's noisy and we have metrics.