photoreal --gpu: fall back, don't refuse (1M-triangle cap, pinhole-only), and decimate for tracing

[ecto]

Two hard refusals hit on rose-pro-skinned (1.5M triangles):

• --gpu errors on >1M triangles ('each triangle costs one 144-byte surface record') and exits without an image instead of falling back to the now-fast CPU path.
• --gpu with --aperture errors (pinhole-only) instead of warning-and-dropping DOF or falling back.

Ask: auto-fallback with a one-line warning in both cases, plus a decimation pre-pass for path tracing — the bare 253-root body traces in ~7 s warm, the skinned one takes ~6 min purely because of skin mesh density the image doesn't need.

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