feat(viewer): stream live pipeline GLBs into the browser viewport

Protocol: RunSnapshot gains CompletedSteps (accumulated from checkpoint
artifact paths, reset on start) so the frontend can tell when real
pipeline geometry exists. New coordinator method StepArtifact plus
GET /api/artifacts/steps serves steps/*.glb while a run is active (any
lifecycle status), traversal-guarded and .glb-only, so stages appear
without waiting for run completion.

Frontend: GLTFLoader loads the latest completed step into the viewport
group as soon as its checkpoint lands over SSE; replay placeholder
frames render only while no step artifacts exist. ?freeze=1 query param
stops rotation for deterministic screenshot verification. Three.js r160
and GLTFLoader (with BufferGeometryUtils) are vendored under public/
vendor with an import map and a window.THREE shim, replacing the CDN
UMD script; the ~1.4MB vendored three.js files are third-party
dependencies (MIT), not project assets.

Tests: store accumulation of completed artifact paths; steps endpoint
serves GLB during an active run, rejects traversal, 404s missing files.

Verified: dotnet build Debug+Release clean, 42/42 tests pass, Fable
compile clean, release server smoke serves index/vendor/steps endpoint.

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