feat: multi-format 3D viewer with large-file OBJ streaming, Z-up, projection/outline UI

Three.js viewer supporting glb/gltf/obj/fbx/dae/ifc/ply via both server (SSR)
and drag&drop (CSR) paths.

- Streaming OBJ parser (src/viewer/objStream.ts) for files past the V8 max
  string length (>~1GB text) that OBJLoader can't handle; indexed geometry,
  per-vertex color from MTL Kd, float64 recenter baked in.
- In-viewer float64 recenter (objRecenter.ts) for huge CAD/survey coordinates
  (~1e8) so float32 vertex buffers keep precision (no cracked faces).
- Z-up right-handed world; Y-up formats rotated on load.
- OBJ+MTL+texture drag&drop (LoadingManager URL-modifier maps dropped images).
- OrbitControls ground-plane panning (road/rail alignment workflow).
- UI: Zoom Fit, perspective/orthographic toggle, feature-edge outline.
- DoubleSide for CAD OBJ; PLY mesh + point-cloud support.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/viewer/hydration.ts

@@ -0,0 +1,35 @@
+// SSR -> CSR hydration gate. Fades the WebP placeholder to the Three.js canvas
+// ONLY after BOTH the WebGL context is ready AND the model is loaded.
+// Firing on just one condition = empty-canvas flash (race). Idempotent.
+
+export interface HydrationGate {
+  markWebGLReady(): void;
+  markModelLoaded(): void;
+}
+
+export function createHydrationGate(preview: HTMLImageElement): HydrationGate {
+  let webglReady = false;
+  let modelLoaded = false;
+  let fired = false;
+
+  function maybeFire(): void {
+    if (fired || !(webglReady && modelLoaded)) return;
+    fired = true;
+    preview.style.opacity = '0';
+    window.setTimeout(() => {
+      preview.classList.add('hidden');
+      preview.style.display = 'none';
+    }, 500);
+  }
+
+  return {
+    markWebGLReady() {
+      webglReady = true;
+      maybeFire();
+    },
+    markModelLoaded() {
+      modelLoaded = true;
+      maybeFire();
+    },
+  };
+}