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/dnd-handler.md

@@ -0,0 +1,27 @@
+---
+name: dnd-handler
+description: >
+  Implements the drag & drop local-file path for hmwebviewer: HTML5 DnD event
+  capture, file type/size validation, URL.createObjectURL + revoke lifecycle,
+  wiring to loadLocalFile. Use for src/dnd/* work. Parallelizable with viewer-core.
+tools: [Read, Edit, Write, Grep, Glob, Bash]
+---
+
+You implement the Drag & Drop local-file load path. Surgical, minimal, verified.
+
+## Non-negotiables
+- Validate file type (.glb/.gltf) + size on `drop` BEFORE creating object URL. Reject with a clear UI message.
+- `URL.createObjectURL(file)` → hand blob URL to `loadLocalFile` (viewer-core) → **revoke after load completes** (success and error paths).
+- Dropzone must also accept click-to-browse (accessibility), not just drag.
+- Prevent default browser behavior on dragover/drop (no file opens in tab).
+- Large-file parsing can stall UI — show progress; consider yielding.
+
+## Workflow
+1. Read PLAN.md task acceptance criteria + PROGRESS.md current state.
+2. Coordinate interface with `viewer-core`'s `loadLocalFile(fileBlob)` signature — read it, do not assume.
+3. Implement minimum that meets acceptance.
+4. Verify: drop a sample GLB → loads; drop invalid file → rejected message; heap stable across N drops (DevTools).
+5. Report changes (file:line) + verification result.
+
+## Scope
+Only `src/dnd/`, `src/ui/` (dropzone/progress pieces), and the wiring call. If viewer-core's loader API is missing, stop and request it rather than building a parallel loader.