feat(isolation): process isolation for custom nodes via pyisolate

Adds opt-in process isolation for custom nodes using pyisolate's
bwrap sandbox and JSON-RPC bridge. Each isolated node pack runs in
its own child process with zero-copy tensor transfer via shared memory.

Core infrastructure:
- CLI flag --use-process-isolation to enable isolation
- Host/child startup fencing via PYISOLATE_CHILD env var
- Manifest-driven node discovery and extension loading
- JSON-RPC bridge between host and child processes
- Shared memory forensics for leak detection

Proxy layer:
- ModelPatcher, CLIP, VAE, and ModelSampling proxies
- Host service proxies (folder_paths, model_management, progress, etc.)
- Proxy base with automatic method forwarding

Execution integration:
- Extension wrapper with V3 hidden param mapping
- Runtime helpers for isolated node execution
- Host policy for node isolation decisions
- Fenced sampler device handling and model ejection parity

Serializers for cross-process data transfer:
- File3D (GLB), PLY (structured + gaussian), NPZ (streaming frames),
  VIDEO (VideoFromFile + VideoFromComponents) serializers
- data_type flag in SerializerRegistry for type-aware dispatch
- Isolated get_temp_directory() fence

New core save nodes:
- SavePLY and SaveNPZ with comfytype registrations (Ply, Npz)

DynamicVRAM compatibility:
- comfy-aimdo early init gated by isolation fence

Tests:
- Integration and policy tests for isolation lifecycle
- Manifest loader, host policy, proxy, and adapter unit tests

Depends on: pyisolate >= 0.9.2

comfy/cli_args.py

@@ -179,6 +179,8 @@ parser.add_argument("--disable-api-nodes", action="store_true", help="Disable lo
 
 parser.add_argument("--multi-user", action="store_true", help="Enables per-user storage.")
 
+parser.add_argument("--use-process-isolation", action="store_true", help="Enable process isolation for custom nodes with pyisolate.yaml manifests.")
+
 parser.add_argument("--verbose", default='INFO', const='DEBUG', nargs="?", choices=['DEBUG', 'INFO', 'WARNING', 'ERROR', 'CRITICAL'], help='Set the logging level')
 parser.add_argument("--log-stdout", action="store_true", help="Send normal process output to stdout instead of stderr (default).")