[BUG] SolverMuJoCo graph-color fallback overflows contype at color 31

[kioxli]

Bug Description

SolverMuJoCo fails to construct a MuJoCo model when its collision-filter graph requires the highest documented supported color.

Newton documents support for 32 graph colors, one per MuJoCo contype bit, and says colors with index >= 32 should fall back to contype=1 / conaffinity=1. Instead, _color_collision_shapes() currently numbers colors from 1. The 31st color is encoded as 1 << 31 == 2147483648, which the MuJoCo Python MjSpec binding rejects because the field is exposed as a signed 32-bit integer.

Consequently, only 30 graph colors are currently safe. A graph requiring 31 colors already fails, and the documented overflow fallback is never reached for larger complete graphs.

Expected behavior:

• Colors 0 through 31 use all 32 MuJoCo mask bits and construct successfully.
• Colors with index >= 32 use the documented contype=1 / conaffinity=1 fallback and the model remains runnable with potentially degraded collision filtering.

Actual behavior:

TypeError: add_geom(): incompatible function arguments
...
contype=2147483648, conaffinity=2147483647

This also remains reachable in the legacy graph-color fallback on #3714. The new collision-mask compiler makes the simple complete-graph reproduction fit without using the fallback, but a graph that exceeds the compiler bounds and requires 31 colors still reaches the unchanged code and fails in the same way.

Related: #3713, #3714.

Reproduction Script

import numpy as np

import newton
from newton.solvers import SolverMuJoCo


builder = newton.ModelBuilder()
for i in range(32):
    body = builder.add_link(label=f"body_{i}")
    builder.add_shape_sphere(body, radius=0.1, label=f"shape_{i}")
    joint = builder.add_joint_free(body)
    builder.add_articulation([joint])

model = builder.finalize(device="cpu")
colors = SolverMuJoCo._color_collision_shapes(model, np.arange(model.shape_count))
print(f"colors: {colors.min()}..{colors.max()}")  # 1..32

SolverMuJoCo(model, use_mujoco_cpu=True)

System Information

• Newton: 1.4.0 and main at ee84505a
• Warp: 1.15.0
• MuJoCo: 3.10.0 (also reproduced with 3.8.1)
• Python: 3.11.15
• OS: Ubuntu Linux 6.8.0-117-generic, x86_64
• GPU: NVIDIA GeForce RTX 5090 D v2, driver 595.71.05 (reproduction uses CPU)

Proposed Fix

• Number graph colors from 0 so bits 0 through 31 are all available.
• Encode the 32-bit mask bit pattern using its signed int32 representation before passing it to MuJoCo (0x80000000 becomes -2147483648).
• Use the full 0xFFFFFFFF affinity mask instead of INT32_MAX.
• Assign contype=1 / conaffinity=1 explicitly for color indices >= 32.
• Add regression coverage for all 32 exact colors and the overflow fallback.