feat(editor): wave-7 polish -- undo UI + toolbar + asset browser (#20)

[havokentity]

Summary

Wave-7 editor polish drop. Three sub-features in one PR:

1. Global keyboard shortcuts

• Cmd/Ctrl+Z dispatches scene_undo
• Cmd/Ctrl+Shift+Z and Ctrl+Y dispatch scene_redo
• G / R / S flip gizmo_mode translate / rotate / scale

Installed by <Shell> so every editor panel gets them automatically. Listener
gates on the focused element: a number-field that wants the user to type s
still works because typing into <input>/<textarea>/contenteditable skips
dispatch.

2. Toolbar panel rebuild (panels/toolbar)

Thin top-bar (~32 px) modeled after Blender / Unity headers:

• Translate / Rotate / Scale -- dispatch gizmo_mode <mode>
• Undo / Redo -- scene_undo / scene_redo (also flash when the matching
  Cmd+Z keyboard shortcut fires)
• Snap toggle (cvar gizmo_snap, new) + Transform-space selector
  (cvar gizmo_space, new; world / local)

Toolbar polls the three cvars every 1 s so external changes (GLFW G/R/S
handler, console, other panels) stay reflected without a manual refresh.

3. Asset Browser panel (panels/asset-browser)

Four tabs, file-path filter, click-to-load:

Tab	Source	Click dispatches
Scenes	tests/goldens/scenes/*.cfg	exec <path>
HDRIs	assets/hdri/*.hdr	r_env_map <path>
glTF	assets/gltf/*.gltf / *.glb	mesh_load_gltf <path>
Meshes	assets/meshes/*.obj / .ply	mesh_load_gltf <path> (stub; no .obj importer yet)

Rows are draggable: drag-start writes the dispatch line into both
application/x-demont-asset (custom MIME) and text/plain so a future
native-viewport drop handler can pick either off the DataTransfer.

Engine additions

• new cvars gizmo_snap (default 0) + gizmo_space (default "world"),
  both CVAR_ARCHIVE so they survive across runs
• new command list_assets <subdir>: directory walk over
  tests/goldens/scenes (.cfg/.toml), assets/hdri (.hdr/.exr),
  assets/gltf (.glb/.gltf), assets/meshes (.obj/.ply/.stl). Emits one
  workspace-relative path per line plus a trailing
  (list_assets: N entries under '<dir>') summary the React client splits on
  \n and drops.

Existing engine commands consumed:

• scene_undo / scene_redo / scene_undo_clear (from wave-6 fix(render): bloom now runs when denoiser is off (Vulkan) #18)
• gizmo_mode cvar (from rotation-gizmo feat(editor): rotation gizmo dispatches prim_set_rotation (orientation for analytic prims) #207 wave)
• exec <cfg> / r_env_map <path> / mesh_load_gltf <path> (existing)

Test plan

• npm run typecheck -- clean (web/editor/)
• npm test -- 41 tests pass (24 hierarchy, 11 keyboard, 6 asset-browser)
• npm run build -- 18 embedded files emitted
• cmake --build build/mac-release --target demont -j 8 -- clean Mac/Metal build
• Smoke: spawn demont, line-TCP probe list_assets scenes -- 29 entries
• Smoke: WS get_cvar gizmo_snap returns {value: "0", default: "0"}; gizmo_space same shape
• Smoke: HTTP GET /editor/{toolbar,asset-browser,inspector,scene-hierarchy} -- 200, <div id="app"> present
• Smoke: WS exec scene_undo on a fresh engine returns scene_undo: history is empty (round-trips properly)

Notes for the integrator

• Test runner now uses happy-dom for the DOM-dependent keyboard handler tests; added as a devDependency in web/editor/package.json
• The Meshes tab is informational only; the engine currently only imports
  glTF. Empty-state hint says so
• gizmo_space local is currently a metadata cvar -- the analytic-prim gizmo
  drag dispatch is world-space only. Wired now so the rotation-aware gizmo
  follow-up (fix(shaders): per-ray underwater medium tracking (Phase 4 #134) #206 -- light / per-prim orientation) can read it without
  another cvar-add roundtrip