feat(workflows): support video model artifacts

[DrHepa]

Closes #345

Summary

• add scalar video input support for model extension nodes
• add a durable Load Video workflow source and video artifact registration
• carry validated video snapshots through Electron, FastAPI, the extension subprocess, and the isolated runner
• preserve exact composite-model pinning and typed artifact dispatch

Dependencies and review scope

This is a stacked PR on top of scene-artifact PR #357 because both use the generic typed /generate/from-artifact route. Until #357 merges, the diff includes its scene commit. The video-only implementation is commit c60d93df.

The Pixal3D consumer also depends on shared-weight PR #348. A temporary integration merge of #348 + #357 + #358 exposed three overlapping validation conflicts (generator_registry.py, extension-install-utils.ts, and ipc-handlers.ts). The combined resolution preserves both artifact validation and node-specific shared-weight projection and passes the integration checks below. After #348 and #357 land, this branch will be rebased onto current dev, those verified resolutions applied, and the PR marked ready.

Scope

This PR deliberately supports scalar model video artifacts. It rejects video arrays, process-extension video I/O, mixed unsupported inputs, traversal, symlink/reparse escapes, file replacement after validation, and oversized files.

Validation

• original branch: 159 Python tests, 30 Node tests, TypeScript --noEmit, electron-vite build, and git diff --check passed
• combined feat(models): support extension-scoped shared weight groups #348 + feat(workflows): support scene model artifacts #357 + feat(workflows): support video model artifacts #358 integration: exact Pixal3D six-node registry projection passed with node-specific shared groups
• combined integration: 100 focused Python tests passed
• combined integration: 9 focused Node suites passed
• combined integration: TypeScript --noEmit passed
• real local video to Pixal3D scene to WorldSculpt GLB generation passed