WIP: Buffer cache: run LRU GC pass when memory threshold is exceeded

[collinmcg]

While checking stutter-related paths, I noticed BufferCache::RunGarbageCollector() was effectively a no-op under pressure.

It calculated thresholds and built a cleanup lambda, but never actually walked the LRU list, so nothing got reclaimed.

This patch does the minimal thing:

• run lru_cache.ForEachItemBelow(gc_tick - ticks_to_destroy, clean_up)
• make the lambda return bool so we can stop after max_deletions

No refactor or behavior changes outside this GC path.

Validation:

• verified call path from Rasterizer::OnSubmit() to RunGarbageCollector()
• local configure succeeds
• full local build still fails in unrelated external code (externals/openal-soft) in this environment
• checked compile of modified TU with compile commands (buffer_cache.cpp)

[red-prig]

Now run tests without using AI on real games and look for differences and regressions.