Cap erode() parameters and run memory guard on every backend (#1275)

[brendancol]

Closes #1275.

Summary

• Caps iterations, params['radius'], and params['max_lifetime'] at the public erode() entry via _validate_scalar(max_val=...). New constants: _MAX_ITERATIONS=1e8, _MAX_RADIUS=1024, _MAX_LIFETIME=1e5. Out-of-range values raise ValueError before any allocation runs.
• Rewrites _check_erosion_memory to include the random_pos buffer and brush arrays in its budget (the old version only counted the raster). Wires it into _erode_numpy and _erode_cupy so the numpy and cupy paths get the guard too, not just dask.
• Mirrors the diffuse fix in Guard diffuse() against unbounded allocations and steps loop (#1267) #1268.

Test plan

• All 15 existing erosion tests still pass
• New: iterations=0, iterations>cap, iterations=10**12, non-int iterations raise ValueError/TypeError before any allocation
• New: radius=0, radius>cap raise ValueError
• New: max_lifetime=0, max_lifetime>cap raise ValueError
• New: monkeypatched _available_memory_bytes makes the numpy and cupy paths raise MemoryError (dask is no longer the only guarded backend)
• New: huge iterations on a tiny raster raises MemoryError mentioning random_pos (the buffer dominates)
• New: defaults still work end-to-end

Deferred follow-ups (per the security-sweep one-fix-per-PR policy)

• Cat 3 HIGH: NaN input is not guarded in _erode_cpu or _erode_gpu_kernel. A NaN cell propagates through bilinear interpolation into dir_x/dir_y, NaN bounds checks fall through, and particles can deposit NaN via cuda.atomic.add into arbitrary cells of the heightmap.
• Cat 6 MEDIUM: erode() does not call _validate_raster(). Non-numeric or wrong-ndim input fails inside numba/cupy with a confusing error.