Add save/load of full particle state (2D & 3D)

[wiesenthal]

What

Adds the ability to save and load the full state of a simulation — the position, velocity and type of every particle — for both the 2D and 3D WebGPU engines. This complements the existing preset system (which only stores rules/radii/colors/settings) by capturing the exact live snapshot, so you can freeze a run and resume it later pixel-for-pixel.

How it works

• composables/useStateManager.ts — serializes a snapshot to a compact binary container and parses it back:

  [4]  magic "SBXS"
  [4]  version (uint32)
  [4]  header length (uint32)
  [..] JSON header (engine, counts, world size, colors, matrices)
  [..] raw Float32 particle data (x,y[,z],vx,vy[,vz],type per particle)
  

  JSON would balloon for millions of particles, so the particle data stays as raw little-endian floats. Includes downloadState() and readStateFile() helpers with toast feedback.

• components/particle-life/StatePanel.vue — a small Save State / Load State panel (hidden file input for load), added to the left sidebar of both engines under a new "Save / Load State" collapse section.

• ParticleLifeGpu.vue / ParticleLifeGpu3D.vue:

  • saveState() reads the live GPU particle buffer back via the existing readBufferFromGPU helper (after onSubmittedWorkDone() so the snapshot is current) and downloads it.
  • loadState(file) parses the file, applies the saved particle/species counts and world size, then rebuilds the simulation — injecting the saved matrices, colors and particle buffer instead of regenerating randomly. This reuses the existing initLife() rebuild path through a small pendingLoadedState seam, so buffer/pipeline recreation stays in one place.
  • Loading a 3D file into the 2D engine (or vice-versa) is rejected with a clear toast.

Buffer layouts handled: 2D = 5 floats/particle [x, y, vx, vy, type], 3D = 7 floats/particle [x, y, z, vx, vy, vz, type].

Testing

• Verified the binary serialize → deserialize roundtrip preserves all particle floats and header metadata exactly, including the offset/subarray case produced by GPU buffer read-back.
• Parsed every changed/added <script> block with oxc-parser to confirm no syntax/TS errors.

Note: a full nuxt build could not be run in my environment due to an unrelated toolchain issue (oxc-walker require()s the ESM-only oxc-parser). Changes are isolated to the feature files and follow the existing engine patterns. A quick manual smoke test (save a running sim, refresh, load it back in both 2D and 3D) is recommended before merge.

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