Add emission curve averaging, render budget tracking, and diagnostics UI

[AlexCat315]

Particle Emitter:

• Add isEmissionActive property: respects duration for non-looping emitters (isEmitting remains for authoring, isEmissionActive reflects runtime state)
• Replace single-point emission curve evaluation with averageMultiplier()
  • Samples curve across delta step (up to 32 samples, 8 per cycle)
  • Prevents long frames from skipping curve features (especially looping)
  • Applies to both continuous emission and distance emission rate curves

RenderScene / Schedule:

• Add sourceParticleCount, submittedSourceParticleCount, renderBudgetSkippedSourceParticleCount to ParticleRenderSummary
• Track source vs submitted particle counts for CPU and GPU batches
• Expose render budget skips separately from active instance count

EditorStatPanel:

• Add Source / Submitted Source / Render Skips to Render Batches group
• Add emitter labels (name, kind, path) to hotspot list and detail
• Add authoring diagnostic rows to selected emitter section
• Add hotspot signal/action/details fields
• Widen hotspot name column, move entity ID to end
• Add ParticleAuthoringDiagnosticsRows component

EditorSceneAdapter:

• hasActiveParticles() uses isEmissionActive instead of isEmitting (non-looping emitters stop driving frames after duration expires)

Tests:

• Add isEmissionActive tests (looping/non-looping)
• Add looping emission curve averaging tests (continuous + distance)
• Add render budget skip diagnostic tests
• Add emitter labels hierarchy path tests
• Add authoring diagnostic tests (GPU blocker, fallback, ready)
• Add hotspot detail/signal/recommendation tests
• Add hotspot severity sort priority test
• Add non-looping particle frame-driving test