Warm likely effects before first use

Depends on !5500 (merged). Related to #4880.

Warms likely one-shot effects before playback. The first pass queues the sound store for hardcoded and scripted sounds. Each cell change prioritizes active-grid region sounds, placed-object sounds, creature sound generators, and animation sound keys.

Effects up to 1 MB are cached whole because playback decodes them on the frame thread. Larger files use the streamed head-cache path. Bulk warming stops near the cache limit; active-grid entries may replace older entries.

Tests: all 510 openmw-tests tests pass locally.

Added a filter on audio file extensions only + a dedupe pass. In total, costs 3ms for a warmup when entering Narsis' cell with the default Wabbajack list installed.

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