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>

packages/nodes/src/stair/definition.ts

@@ -0,0 +1,59 @@
+import { type NodeDefinition, StairNode as StairNodeSchema } from '@pascal-app/core'
+import { buildStairFloorplan } from './floorplan'
+import { stairParametrics } from './parametrics'
+import { StairNode } from './schema'
+
+/**
+ * Stair — Stage A. Composite node like roof: owns overall framing,
+ * `stair-segment` children own per-flight geometry. Wrap-exports the
+ * legacy `StairRenderer` + `StairSystem`.
+ */
+export const stairDefinition: NodeDefinition<typeof StairNode> = {
+  kind: 'stair',
+  schemaVersion: 1,
+  schema: StairNode,
+  category: 'structure',
+
+  defaults: () => {
+    const stub = StairNodeSchema.parse({ id: 'stair_default' as never, type: 'stair' })
+    const { id: _id, type: _type, ...rest } = stub
+    return rest
+  },
+
+  capabilities: {
+    selectable: { hitVolume: 'bbox' },
+    duplicable: true,
+    deletable: true,
+  },
+
+  parametrics: stairParametrics,
+
+  renderer: {
+    kind: 'parametric',
+    module: () => import('./renderer'),
+  },
+  system: {
+    module: () => import('./system'),
+    priority: 3,
+  },
+  // Stage C — stair is the parent; it walks its `stair-segment` children
+  // via `ctx.children` and emits the whole stack as one registry entry.
+  // Each flight's transform depends on every prior sibling's
+  // `length` / `height` / `attachmentSide`, so individual segments can't
+  // compute their own polygon in isolation. See
+  // `nodes/src/stair/floorplan.ts` for the emitter.
+  floorplan: buildStairFloorplan,
+
+  presentation: {
+    label: 'Stair',
+    description:
+      'A stair composed of one or more flights with configurable treads, risers, railings.',
+    icon: { kind: 'url', src: '/icons/stairs.png' },
+    paletteSection: 'structure',
+    paletteOrder: 110,
+  },
+
+  mcp: {
+    description: 'A multi-flight stair with segmented geometry.',
+  },
+}