ParticleRenderer.Draw is called up to ~11 times per frame (sky pre/post,
scene, per-visible-cell, dynamics, unattached passes). Each call
allocated a fresh List<ParticleDraw> (BuildDrawList) and a fresh
List<ParticleInstance> (the per-batch `run` list), and ParticleSystem.
EnumerateLive was a `yield return` iterator block - a heap-allocated
state machine allocated fresh on every Draw call regardless of particle
count.
Fix: reuse _drawListScratch/_runScratch fields (Clear() + refill) on
ParticleRenderer. Replace EnumerateLive's iterator with a struct
enumerable/enumerator pair (ParticleSystem.LiveParticleEnumerable): the
foreach fast path uses the struct enumerator directly (zero allocation),
while LINQ/test callers (.ToList(), .Single(), etc.) still work via the
explicit IEnumerable<T> interface implementation, which falls back to a
boxed iterator only when that surface is used - those call sites are
test-only, not the per-frame render path this task targets.
Verified safe: all 11 Draw() call sites in GameWindow.cs are plain
synchronous invocations from the single-threaded OnRender chain (no
Task.Run/Parallel dispatch), and each call fully drains its lists
before returning - no call ever overlaps another's use of the reused
buffers. Per-cell filtering semantics unchanged (same predicate, same
traversal order).
dotnet build clean, dotnet test 4120 passed / 4 skipped (unchanged).
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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