Phase 5 Stage E: fence / slab / ceiling drop their legacy panels

Three flavors of Stage E in one PR:

- **Fence** — fully auto-derived. Adds `kind: 'boolean'` to ParamField
  (rendered as ToggleControl), wires `showInfill` through
  `def.parametrics`. Legacy `FencePanel` deleted; the auto-derived
  `<ParametricInspector>` now drives fence editing entirely.

- **Slab / Ceiling** — kind-owned via `parametrics.customPanel`. The
  legacy panels have shape-specific bits (elevation/height presets,
  area display, holes list with auto-vs-manual provenance) that don't
  fit the auto-derived field model yet. `<ParametricInspector>` learns
  to lazy-load and mount `parametrics.customPanel` when present;
  legacy `SlabPanel` + `CeilingPanel` files relocate to
  `nodes/src/<kind>/panel.tsx` and the legacy copies delete.

  When `list` / `computed` / `action` field kinds eventually graduate
  to auto-derived support, these custom panels collapse back into
  `parametrics.groups`. The plan calls this out under "Custom-behavior
  escape hatch" and Foot-gun 5 of the recipe.

Public-surface additions in `@pascal-app/editor`:
- `ActionButton`, `ActionGroup`, `PanelSection`, `SegmentedControl`,
  `ToggleControl`, `PanelWrapper` — needed by the kind-owned panels.

Per-kind progress table: fence/slab/ceiling all flip to E ✅.

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

packages/nodes/src/fence/parametrics.ts

@@ -34,6 +34,7 @@ export const fenceParametrics: ParametricDescriptor<FenceNode> = {
       fields: [
         { key: 'style', kind: 'enum', options: ['slat', 'rail', 'privacy'] },
         { key: 'baseStyle', kind: 'enum', options: ['floating', 'grounded'] },
+        { key: 'showInfill', kind: 'boolean' },
         { key: 'color', kind: 'color' },
       ],
     },