Phase 5 Stage E: full kind migration into packages/nodes

Wholesale move of every remaining kind into its own subdirectory under
`packages/nodes/src/`, finishing the registry-driven migration. Each
kind now ships its definition, schema (re-exported from core), and any
of `geometry` / `renderer` / `system` / `floorplan` / `tool` /
`move-tool` / `panel` / `floorplan-move` / `floorplan-affordances` /
`parametrics` / `preview` it needs — no per-kind code remains under
`packages/editor/src/components/tools/` or
`packages/viewer/src/components/renderers/`.

Deleted (replaced by registry-driven equivalents):
- `tools/{ceiling,column,door,fence,item,slab,spawn,wall,window}/...`
  (boundary editors, hole editors, placement tools, move tools,
  endpoint movers, curve tools, helpers, math libs)
- `ui/helpers/{ceiling,slab,wall}-helper.tsx`
- `ui/panels/{column,door,elevator,item,roof,roof-segment,spawn,
  stair,stair-segment,wall,window}-panel.tsx`
- `viewer/src/components/renderers/{building,ceiling,column,door,
  elevator,fence,guide,item,level,roof,roof-segment,scan,site,slab,
  spawn,stair,stair-segment,wall,window,zone}-renderer.tsx`
- `viewer/src/components/viewer/legacy-system.tsx`

Added under `packages/nodes/src/`:
- `building/`, `column/`, `elevator/`, `guide/`, `level/`, `roof/`,
  `roof-segment/`, `scan/`, `shared/`, `site/`, `stair/`,
  `stair-segment/` packages with definition + schema + renderer / system
  / floorplan / panel as appropriate.
- New `floorplan-move.ts` for every kind that supports 2D moves
  (ceiling, door, item, shelf, slab, window) — single registry-driven
  dispatch path via `def.floorplanMoveTarget`.
- New `floorplan-affordances.ts` for kinds with polygon / endpoint
  drags (ceiling, fence, slab, wall) — using the shared
  `polygon-vertex-affordance` factories.
- New per-kind `panel.tsx` for kinds with custom inspector content
  (door, item, shelf, spawn, wall, window).
- New per-kind `tool.tsx` for placement (door, item, shelf, window).
- New per-kind `move-tool.tsx` for kinds with custom 3D move flows
  (door, item, slab, window).

Coordinator + manager updates in `packages/editor/`:
- `tool-manager.tsx` resolves tools from the registry only — no
  hardcoded type→component map.
- `panel-manager.tsx` resolves inspector panels the same way.
- `placement-{coordinator,strategies,types}.ts` extended with
  shelf-surface placement.
- `selection-manager.tsx` adds the registry-selectable fallback.
- `floorplan-panel.tsx`, `floorplan-background-placement.ts`,
  `floorplan-render-context.tsx` updated for the registry layer's new
  contract (props, affordance dispatch, render context).

Viewer updates:
- `viewer/index.tsx` drops legacy renderer mounts.
- `node-renderer.tsx` resolves by registry only.
- `scene-bvh.tsx`, `use-node-events.ts`, `level-system.tsx`,
  `wall-cutout.tsx`, `zone-system.tsx`, `materials.ts` adjusted for
  the registry-only world.

Sidebar tree nodes for ceiling / fence / slab / shelf / tree-node
updated to read from the registered nodes instead of the deleted
legacy renderer trees.

Wiki: new `plugin-authoring.md` page, README index updated.

Tests in `packages/nodes/src/index.test.ts` validate every registered
kind has the required shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/editor/lib/bootstrap.ts

@@ -1,4 +1,4 @@
-import { loadPlugin, nodeRegistry } from '@pascal-app/core'
+import { discoverPlugins, loadPlugin, nodeRegistry } from '@pascal-app/core'
 import { builtinPlugin } from '@pascal-app/nodes'
 
 // Idempotency guard: HMR can reload this module, but `registerNode` throws on
@@ -12,10 +12,19 @@ function isDev(): boolean {
   return env?.NODE_ENV !== 'production'
 }
 
-export function loadBuiltinNodes(): void {
+export async function loadBuiltinNodes(): Promise<void> {
   if (loaded) return
   loaded = true
-  void loadPlugin(builtinPlugin)
+  await loadPlugin(builtinPlugin)
+
+  // Phase 6 plugin discovery hook. Always called; default impl returns
+  // `[]`. Apps that ship external node packs override the discovery via
+  // `setPluginDiscovery(...)` before this module loads. See
+  // `wiki/editor-plugin-authoring.md` for the contract.
+  const externals = await discoverPlugins()
+  for (const plugin of externals) {
+    await loadPlugin(plugin)
+  }
 
   if (isDev()) {
     const kinds = Array.from(nodeRegistry.entries(), ([k]) => k)
@@ -24,7 +33,7 @@ export function loadBuiltinNodes(): void {
       // "which path is running this kind?" Empty array = every kind is on
       // the legacy path. Kind in the array = registry path is live for it.
       console.info(
-        `[pascal:registry] loaded ${builtinPlugin.id} v${builtinPlugin.apiVersion} (${kinds.length} kinds: ${kinds.join(', ') || '∅'})`,
+        `[pascal:registry] loaded ${builtinPlugin.id} v${builtinPlugin.apiVersion} (${kinds.length} kinds: ${kinds.join(', ') || '∅'})${externals.length > 0 ? ` + ${externals.length} discovered plugin(s)` : ''}`,
       )
     }
     // Expose the registry on window for ad-hoc dev inspection. In prod the
@@ -38,4 +47,4 @@ export function loadBuiltinNodes(): void {
 
 // Run as a side effect on first import so any consumer of this module gets a
 // populated registry without remembering to call the function explicitly.
-loadBuiltinNodes()
+void loadBuiltinNodes()