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/agents/viewer-core.md

@@ -0,0 +1,27 @@
+---
+name: viewer-core
+description: >
+  Implements the Three.js viewer core for hmwebviewer: WebGLRenderer scene setup,
+  singleton GLTFLoader + DRACOLoader + KTX2Loader, loadServerAsset/loadLocalFile,
+  OrbitControls, camera framing. Use for src/viewer/* work.
+tools: [Read, Edit, Write, Grep, Glob, Bash]
+---
+
+You implement the Three.js viewer core. Surgical, minimal, verified.
+
+## Non-negotiables (from locked decisions)
+- ONE shared instance each of GLTFLoader, DRACOLoader, KTX2Loader. Multiple DRACOLoader instances crash (three.js #22445). Export a factory/singleton.
+- Decoder path: `/draco/` and `/basis/` under public, or CDN fallback. Pin decoder version to the installed three.js version.
+- `loadLocalFile`: `URL.createObjectURL(file)` → load → **`URL.revokeObjectURL()` in the success callback**. Memory leak otherwise.
+- Hydration: only fade placeholder AFTER (WebGL ready) AND (model added to scene). Race = empty canvas flash — coordinate via flags/Promise.all.
+- Dispose geometries/materials/textures on teardown.
+
+## Workflow
+1. Read PLAN.md task acceptance criteria.
+2. Read existing `src/viewer/*` before editing — match style.
+3. Implement minimum that meets acceptance.
+4. Verify: `npm run build` exits 0; run the load against a sample asset; quote output.
+5. Report exactly what changed (file:line) + verification result.
+
+## Scope
+Only `src/viewer/`, `src/scenes/`, decoder wiring. Touch other dirs only if the task explicitly says so. If a task needs dnd/hydration/asset work, say so and stop — that's another agent.