RFC-0040: Soft bodies (mass-spring grids)

Status: Normative. The language gains soft bodies: an nx × ny grid of dynamic particles connected by spring constraints, solved by the same deterministic, mass-weighted position relaxation as joints (RFC-0039). Soft bodies act on the EIR component surface (transform positions, velocity, rigid_body mass) and reuse only existing opcodes, so they compose with gravity/integrate and remain byte-identical across the interpreter and JIT.

Declaration

world {
  soft cloth {
    nx = 8; ny = 8
    spacing = 0.25
    origin = (0, 4, 0)
    mass = 0.2
    shape = sphere; size = 0.08
  }
}
systems {
  gravity   { gravity_y = -9.81; dt = 0.016 }
  integrate { dt = 0.016 }
  soft      { body = cloth; stiffness = 1.0; damping = 0.5; iterations = 6 }
}

Fields (all optional except a sensible default): nx (default 8), ny (default 8), spacing (1.0), origin (0,0,0), mass (1.0), and the presentation shape/size applied to every particle.

Rules

1. Particles. A soft declaration creates nx × ny × nz active dynamic entities (nz defaults to 1, a sheet), ids contiguous and immediately after the declared entities, channels, and pools (matching build_scene); particle k = (kz·ny + ky)·nx + kx is named <body>#<k> and starts at origin + (i·spacing, j·spacing, k·spacing) with mass and a zero velocity.
2. Constraints. The grid generates:
   • structural edges one cell along each axis (spacing);
   • shear edges on both diagonals of each coordinate plane (spacing·√2);
   • bend edges two cells along each axis (2·spacing). With nz = 1 the edges involving the z axis vanish, leaving the 2D sheet. Each is a distance constraint |p_a − p_b| = rest.
3. Lowering. A soft system lowers one function per particle. The particle owning a constraint's lower index applies it; per iteration it applies every such constraint as a position-relaxation pass (the RFC-0039 distance math, mass-weighted by is_dynamic ? 1/mass : 0), then the next particle's function applies its own. iterations (default 4) repeats the per-particle sweep; stiffness and optional damping (along-axis relative velocity) parameterize the springs.
4. Determinism. The sweep order is fixed (particle id, then constraint order, then iterations); every read/write is an existing opcode, so results are reproducible and identical on both backends. No new opcode, ABI, schema, protocol, snapshot, or transaction contract is introduced.
5. Presentation. Particles render as their shape; the structural and shear edges are emitted as PresentationFrame.bonds, so the viewer draws the mesh.
6. Scope. This is a lattice of point masses with distance springs: nz = 1 models cloth/membrane sheets, nz > 1 a soft gel cube. Volume/pressure constraints and self-collision are out of scope; soft complements joint/pool and does not replace a full FEM solver.

Implementation is staged: Stage 1 adds the soft declaration, particle layout, and the soft system (structural/shear/bend, position relaxation). Stage 2 adds render bonds and the example/docs. Stage 3 re-baselines conformance and the example programs.