feat(particles): particle system feature set (P0-P2)
Rendering: rotation render fix, additive/opaque blend modes, flipbook animation, gradients and lifetime curves, unified DrawItem2D render path (Vulkan + WebGPU).
Simulation: timed bursts, prewarm, line/ring shapes, gravity, damping, velocity limits/curves, inherit velocity, local/world space, collision (bounds + resolver), CPU noise, sub-emitters, stretched trails, overflow policies, dt caps, offscreen sim skip.
Advanced: radial force fields, custom particle shaders, pooled Light2D particle lights, opt-in GPU-accelerated simulation with automatic CPU fallback.
Fixes: Vulkan pipeline builder lifetime bug (crash on window init), emitter-life emission timing, emit() syncs attachment position.
Tests: new coverage incl. scale regression (128 emitters).
西风逍遥游
d71b126636