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>

.claude/skills/threejs-viewer/SKILL.md

@@ -0,0 +1,89 @@
+---
+name: threejs-viewer
+description: >
+  Domain knowledge for building the hmwebviewer Three.js 3D viewer. Load when working
+  on src/viewer, src/dnd, src/ui, the asset pipeline, or hydration. Covers the locked
+  technical decisions, loader setup, Draco/KTX2, Blob URL lifecycle, and SSR hydration
+  patterns specific to this project.
+---
+
+# hmwebviewer — Three.js 3D viewer domain guide
+
+Source spec: `3d_viewer_architecture_spec.pdf`. Always also read `CLAUDE.md` + `PLAN.md`.
+
+## Locked stack
+- Three.js + Vite + TypeScript.
+- Loaders: `GLTFLoader` + `DRACOLoader` + `KTX2Loader`.
+- Compression: Draco (geometry) + KTX2/Basis Universal (textures).
+
+## Critical patterns (do not deviate without approval)
+
+### Single shared loader instances
+Multiple simultaneous `DRACOLoader` instances crash (three.js #22445). KTX2 same risk. Build a singleton:
+
+```ts
+// src/viewer/loaders.ts
+import * as THREE from 'three';
+import { GLTFLoader } from 'three/examples/jsm/loaders/GLTFLoader.js';
+import { DRACOLoader } from 'three/examples/jsm/loaders/DRACOLoader.js';
+import { KTX2Loader } from 'three/examples/jsm/loaders/KTX2Loader.js';
+
+let _gltf: GLTFLoader | null = null;
+export function getLoaders(renderer: THREE.WebGLRenderer) {
+  if (!_gltf) {
+    const draco = new DRACOLoader().setDecoderPath('/draco/');
+    const ktx2  = new KTX2Loader().setTranscoderPath('/basis/').detectSupport(renderer);
+    _gltf = new GLTFLoader().setDRACOLoader(draco).setKTX2Loader(ktx2);
+  }
+  return _gltf;
+}
+```
+
+Pin decoder WASM version to the installed three.js version. Mismatch → silent decode failures.
+
+### Local file load (Blob URL lifecycle)
+```ts
+const url = URL.createObjectURL(file);
+loader.load(url, (gltf) => {
+  scene.add(gltf.scene);
+  URL.revokeObjectURL(url);          // success → revoke
+}, undefined, (err) => {
+  URL.revokeObjectURL(url);          // error → also revoke
+  throw err;
+});
+```
+For repeat/cached loads: `FileReader` → `ArrayBuffer` → `GLTFLoader.parse()` enables IndexedDB caching and avoids URL overhead.
+
+### Server asset (SSR + CSR + hydration)
+1. SSR ships HTML/CSS skeleton + pre-rendered 360° animated WebP placeholder.
+2. CSR background: `GLTFLoader.load(serverUrl, onLoad, onProgress)`.
+3. Hydration: gate the fade on BOTH `webglReady` AND `modelLoaded` (Promise.all). Fade CSS `opacity 0.5s ease` → `display:none` after 500ms. Race → empty canvas flash.
+
+### Drag & drop
+```ts
+dropzone.addEventListener('dragover', (e) => { e.preventDefault(); });
+dropzone.addEventListener('drop', (e) => {
+  e.preventDefault();
+  const file = e.dataTransfer?.files?.[0];
+  if (!file) return;
+  if (!/\.gl[bt]f$/i.test(file.name)) return showError('GLB/GLTF only');
+  loadLocalFile(file);   // viewer-core, handles createObjectURL/revoke
+});
+```
+Also wire click→`<input type=file">` for accessibility.
+
+### Disposal
+On teardown / model swap: `geometry.dispose()`, `material.dispose()` (and material maps), `texture.dispose()`. Revoke any lingering object URLs.
+
+## Offline asset pipeline
+- Compress: `gltf-transform` CLI or programmatic functions — Draco + KTX2 in one pass.
+- KTX2 encoder binary `toktx` must be on PATH (gltf-transform fetches via `@ktx2/basis-transcoder`/platform binaries — confirm available).
+- Pre-render 360°: Blender headless `blender -b scene.blend -o //frame_### -f 1..N -F PNG` → ffmpeg → animated WebP. Fallback: Puppeteer + headless three.js screenshot per rotation.
+
+## Common pitfalls
+- `setDecoderPath` wrong → worker fetch 404 in console. Verify `/draco/` resolves under `public/`.
+- Forgetting `e.preventDefault()` on dragover → browser opens the file.
+- Decoder version mismatch → model fails to decode with no obvious error.
+- Creating loaders per load → intermittent crashes + memory growth.
+
+References in user memory `reference-threejs-resources.md`.