parametrics: add custom field kind + restore fence Length / Curve

User noticed the auto-derived fence inspector was missing the legacy
panel's Length + Curve sliders, and the section labelling was wrong
(Posts → Structure). Both Length and Curve are awkward for the
parametrics field model:

- **Length** doesn't map to a single node key — it's derived from
  `start`/`end`, and editing it moves `end` along the existing
  direction.
- **Curve** maps to `curveOffset` but the slider's min/max are
  bounded per-node by the chord length, plus updates need
  `normalizeWallCurveOffset`.

Adds a `kind: 'custom'` field with a kind-supplied
`component: ComponentType<{ node, onUpdate }>`. The inspector mounts
it and lets the kind own rendering + update logic. `key` becomes a
free-form React key/label since it no longer needs to map to a node
property.

Fence parametrics now mirrors the legacy layout 1:1:
- Style (segmented controls + showInfill toggle).
- Dimensions (Length, Curve, Height, Thickness).
- Structure (Base Height, Top Rail, Post Spacing, Post Size, Ground
  Clear, Edge Inset).

Length + Curve live in fence/inspector-editors.tsx.

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

packages/nodes/src/fence/parametrics.ts

@@ -1,34 +1,21 @@
 import type { ParametricDescriptor } from '@pascal-app/core'
+import { FenceCurveEditor, FenceLengthEditor } from './inspector-editors'
 import type { FenceNode } from './schema'
 
 /**
- * Inspector descriptor for fence.
+ * Inspector descriptor for fence. Mirrors the legacy `FencePanel`
+ * layout 1:1:
+ *  - **Style** (segmented controls): style, baseStyle, showInfill toggle.
+ *  - **Dimensions**: Length (derived from start/end), Curve (sagitta
+ *    with dynamic bounds), Height, Thickness.
+ *  - **Structure**: Base Height, Top Rail, Post Spacing, Post Size,
+ *    Ground Clear, Edge Inset.
  *
- * Mirrors the legacy `fence-panel.tsx` controls but rendered by the
- * generic `<ParametricInspector>`. Endpoints (`start` / `end`) and
- * `curveOffset` are edited via floor-plan affordances and 3D handles,
- * not number inputs — kept out of parametrics.
+ * Length + Curve use the `custom` field kind because they don't map
+ * to single number fields with static bounds — see `inspector-editors.tsx`.
  */
 export const fenceParametrics: ParametricDescriptor<FenceNode> = {
   groups: [
-    {
-      label: 'Dimensions',
-      fields: [
-        { key: 'height', kind: 'number', unit: 'm', min: 0.4, max: 3.5, step: 0.05 },
-        { key: 'thickness', kind: 'number', unit: 'm', min: 0.02, max: 0.3, step: 0.005 },
-        { key: 'baseHeight', kind: 'number', unit: 'm', min: 0, max: 0.6, step: 0.01 },
-        { key: 'groundClearance', kind: 'number', unit: 'm', min: 0, max: 0.5, step: 0.01 },
-      ],
-    },
-    {
-      label: 'Posts',
-      fields: [
-        { key: 'postSpacing', kind: 'number', unit: 'm', min: 0.5, max: 5, step: 0.1 },
-        { key: 'postSize', kind: 'number', unit: 'm', min: 0.04, max: 0.4, step: 0.01 },
-        { key: 'topRailHeight', kind: 'number', unit: 'm', min: 0, max: 0.2, step: 0.005 },
-        { key: 'edgeInset', kind: 'number', unit: 'm', min: 0, max: 0.1, step: 0.005 },
-      ],
-    },
     {
       label: 'Style',
       fields: [
@@ -47,5 +34,25 @@ export const fenceParametrics: ParametricDescriptor<FenceNode> = {
         { key: 'showInfill', kind: 'boolean' },
       ],
     },
+    {
+      label: 'Dimensions',
+      fields: [
+        { key: 'length', kind: 'custom', component: FenceLengthEditor },
+        { key: 'curve', kind: 'custom', component: FenceCurveEditor },
+        { key: 'height', kind: 'number', unit: 'm', min: 0.4, max: 4, step: 0.05 },
+        { key: 'thickness', kind: 'number', unit: 'm', min: 0.03, max: 0.5, step: 0.005 },
+      ],
+    },
+    {
+      label: 'Structure',
+      fields: [
+        { key: 'baseHeight', kind: 'number', unit: 'm', min: 0.04, max: 1, step: 0.01 },
+        { key: 'topRailHeight', kind: 'number', unit: 'm', min: 0.01, max: 0.25, step: 0.005 },
+        { key: 'postSpacing', kind: 'number', unit: 'm', min: 0.2, max: 5, step: 0.05 },
+        { key: 'postSize', kind: 'number', unit: 'm', min: 0.01, max: 0.4, step: 0.005 },
+        { key: 'groundClearance', kind: 'number', unit: 'm', min: 0, max: 0.6, step: 0.005 },
+        { key: 'edgeInset', kind: 'number', unit: 'm', min: 0.005, max: 0.25, step: 0.005 },
+      ],
+    },
   ],
 }