[chemistry] Replace L-systems with IRL reaction-diffusion

[M1thieu]

Status: HIGH PRIORITY after MPM

Reaction-diffusion is a natural fit for MPM grid-based architecture.

Why High Priority

1. Naturally grid-based (matches MPM grid)
2. MPM particles carry chemical concentrations
3. Diffusion via P2G/G2P transfers
4. Reactions at particle level

MPM-Based Implementation

Particle structure:

struct MPMParticle {
    position: Vec3,
    velocity: Vec3,
    mass: f32,
    deformation_gradient: Mat3,
    concentrations: Vec<f32>, // [O2, H2O, glucose, ...]
}

Diffusion via P2G/G2P, reactions at particle level.

Acceptance Criteria

• BLOCKER: MPM solver working
• Particles carry concentration vector
• P2G transfers chemical mass to grid
• Grid diffusion (Fick's law)
• G2P transfers chemicals to particles
• Test: Turing patterns emerge

Implement after basic MPM working.