Instrument CachingReader starvation diagnostics

[Kybernetria]

Summary

• replace per-callback request-submission warning spam with a five-second, per-deck diagnostic report
• track request submissions/dequeues, status publications/consumption, cache misses, and worker progress using lock-free counters
• identify request saturation, suspected decoder stalls, and status backpressure
• include FIFO occupancy/capacity, active/completed/failed chunks, and recovery reports
• keep all formatting and logging off the realtime engine callback

This is instrumentation-only: it does not change cache admission, decoding, or silence behavior. Field output will determine the appropriate follow-up fix.

Validation

• clang-format 19.1.3 via tools/clang_format.py
• git diff --check
• built mixxx-test successfully with CCACHE_DISABLE=1
• 7 focused ReadAheadManager/Bungee tests passed

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