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.

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/SomhairlesDream.Server.Tests/StoreTests.fs b/tests/SomhairlesDream.Server.Tests/StoreTests.fs
index 4aec16f..5ef2cba 100644
--- a/tests/SomhairlesDream.Server.Tests/StoreTests.fs
+++ b/tests/SomhairlesDream.Server.Tests/StoreTests.fs
@@ -90,6 +90,38 @@ module StoreTests =
         Assert.False(snapshot.IsStale)
 
     [<Fact>]
+    let ``artifact store accumulates completed step artifact paths`` () =
+        let store = ArtifactRunStore(artifactIds, TimeSpan.FromSeconds 15., at 9)
+
+        store.Apply(
+            RunEvent.Start
+                { Ids = artifactIds
+                  At = at 9
+                  TotalSteps = 2 }
+        )
+        |> ignore
+
+        Assert.Empty(store.Snapshot().CompletedSteps)
+
+        for index, stepId in [| 0, "01-foundation"; 1, "02-frame" |] do
+            store.Apply(
+                RunEvent.Checkpoint
+                    { Ids = artifactIds
+                      At = at 9
+                      StepId = stepId
+                      StepIndex = index
+                      TotalSteps = 2
+                      Message = "artifact verified"
+                      ArtifactPath = $"steps/{stepId}.glb" }
+            )
+            |> ignore
+
+        Assert.Equal<string array>(
+            [| "steps/01-foundation.glb"; "steps/02-frame.glb" |],
+            store.Snapshot().CompletedSteps
+        )
+
+    [<Fact>]
     let ``artifact store derives staleness without changing lifecycle status`` () =
         let store = ArtifactRunStore(artifactIds, TimeSpan.FromSeconds 15., at 9)