Add Pixal3D Apple Silicon image-to-3D target

[atomantic]

Summary

• Registers Pixal3D (Apple Silicon) alongside TRELLIS.2 in the generic image-to-3D target selector.
• Adds an isolated Python 3.10 MPS/Metal runtime using the community Pixal3D-mac port.
• Keeps TRELLIS.2 as the default target and starts Pixal3D with the safer 1024 cascade.
• Preserves the no-cold-bootstrap contract: installation and inference happen only after explicit user actions.

Implementation

• Adds idempotent clone, Python 3.10, Torch/Torchvision, NATTEN, and native Metal package setup.
• Adds module probing and degraded-install diagnostics so incomplete native builds do not appear ready.
• Streams the fork progress banners into the shared render progress contract.
• Classifies Metal watchdog, unified-memory exhaustion, and Hugging Face authentication failures with actionable errors.
• Keeps the target gated to Apple Silicon with a 24 GB unified-memory floor and a 40 GB disk budget.

Validation

• Focused post-review suite: 4 files, 76 tests passed.
• Full server suite: 32,834 tests passed, 19 skipped.
• Full client suite: 9,369 tests passed.
• Client production build passed.
• Live MPS module probe found all required native modules.
• End-to-end adapter smoke render on a public sample produced a valid binary glTF v2 GLB through the PortOS runner.

Notes

The first explicit render downloads model weights and may take several minutes. The Apple port and its public dependency mirrors are linked from the target card; no gated model approval is required for this lane.