feat(ui): custom modifiers in modifier profile panels

T3 Wave 4 (T26). PerTypePanel and PerInstancePanel kind dropdowns now
include the user's custom modifier library alongside built-ins.

- Kind dropdown uses <optgroup> separation: 'Built-in' (MODIFIER_REGISTRY)
  + 'Custom (from library)' (loadCustomModifierLibrary).
- Selecting a custom kind hides the per-instance value input and shows
  a small summary card ('Custom modifier — N primitives. Edit it in
  the Custom Modifier editor.') — the descriptor's primitive list IS
  the payload; per-instance value is null.
- Custom kinds skip Zod-schema-based validation (they have no
  per-instance value to validate); isValid is true once a custom
  kind is selected.
- handleSave / handleAdd branches on isCustomKind: built-in path
  preserves T1/T2 behaviour exactly; custom path emits {kind: id,
  pieceType, color, value: null}.
- PerInstancePanel mirrors the same pattern in AddModifierForm.

E2E tests for the new UI defer to T29; existing 1378 unit tests pass.

packages/chess/src/ui/PerInstancePanel.tsx

@@ -11,11 +11,15 @@
  *   3. For the selected square → view existing instance modifiers and
  *      add/delete them via the right-hand sidebar form.
  */
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 import type { InstanceModifier, ModifierKindId } from '../modifiers/types';
 import type { StartingLayout } from '../layouts/types';
 import { squareToAlgebraic } from '../coord';
 import { LayoutBoardView } from './LayoutEditor';
+import {
+  loadCustomModifierLibrary,
+  type SavedCustomModifier,
+} from '../modifiers/custom/library.js';
 import type { ModifierClipboard } from './ModifierProfileEditor.js';
 
 export interface PerInstancePanelProps {
@@ -264,10 +268,29 @@ function AddModifierForm({
 }) {
   const [kind, setKind] = useState<ModifierKindId>('hp-bonus');
   const [rawValue, setRawValue] = useState<string>('0');
+  // Custom modifiers from the user's library appear in the kind
+  // dropdown alongside built-ins (T26). Reload on mount; new entries
+  // saved via CustomModifierEditor become visible the next time the
+  // user opens this form.
+  const [customs, setCustoms] = useState<SavedCustomModifier[]>(() =>
+    loadCustomModifierLibrary(),
+  );
+  useEffect(() => {
+    setCustoms(loadCustomModifierLibrary());
+  }, []);
+
+  const customDescriptor = customs.find((c) => c.id === kind);
+  const isCustomKind = customDescriptor !== undefined;
 
   function handleAdd() {
-    let parsed: unknown;
+    if (isCustomKind && customDescriptor) {
+      // Custom kinds carry no per-instance value; the descriptor's
+      // primitive list is the entire payload.
+      onAdd({ kind: customDescriptor.id, square, value: null });
+      return;
+    }
 
+    let parsed: unknown;
     if (kind === 'direction-additions') {
       parsed = rawValue
         .split(',')
@@ -301,14 +324,41 @@ function AddModifierForm({
         }}
         className="w-full px-3 py-2 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-blue-500"
       >
+        <optgroup label="Built-in">
           {MODIFIER_KINDS.map(({ id, label }) => (
             <option key={id} value={id}>
               {label}
             </option>
           ))}
+        </optgroup>
+        {customs.length > 0 && (
+          <optgroup label="Custom (from library)">
+            {customs.map((c) => (
+              <option
+                key={c.id}
+                value={c.id}
+                data-testid={`instance-kind-custom-${c.id}`}
+              >
+                {c.descriptor.name}
+              </option>
+            ))}
+          </optgroup>
+        )}
       </select>
 
+      {isCustomKind && customDescriptor !== undefined ? (
+        <div
+          data-testid="instance-custom-modifier-summary"
+          className="px-3 py-2 text-xs text-neutral-600 bg-neutral-50 border border-neutral-200 rounded"
+        >
+          Custom modifier — {customDescriptor.descriptor.primitives.length}{' '}
+          primitive
+          {customDescriptor.descriptor.primitives.length !== 1 ? 's' : ''}.
+          Edit it in the Custom Modifier editor.
+        </div>
+      ) : (
         <ValueInput kind={kind} value={rawValue} onChange={setRawValue} />
+      )}
 
       <button
         data-testid="instance-modifier-add"

packages/chess/src/ui/PerTypePanel.tsx

@@ -11,10 +11,14 @@
  *   "direction-set"    → placeholder (T21+)
  *   "capture-flags"    → placeholder (T21+)
  */
-import { useState } from 'react';
+import { useEffect, useState } from 'react';
 import { MODIFIER_REGISTRY } from '../modifiers/index.js';
 import type { TypeModifier, ModifierDescriptor } from '../modifiers/types.js';
 import type { PieceType, PieceColor } from '../schema.js';
+import {
+  loadCustomModifierLibrary,
+  type SavedCustomModifier,
+} from '../modifiers/custom/library.js';
 import type { ModifierClipboard } from './ModifierProfileEditor.js';
 
 const PIECE_TYPES: PieceType[] = [
@@ -63,13 +67,26 @@ export function PerTypePanel({ modifiers, clipboard, onClipboardChange, onPaste,
   const [color, setColor] = useState<PieceColor | 'both'>('both');
 
   const descriptors = MODIFIER_REGISTRY.list();
+  // Custom modifiers from the user's library appear in the kind
+  // dropdown alongside built-ins (T26). Re-loaded when the form
+  // opens so newly-saved custom modifiers show up immediately.
+  const [customs, setCustoms] = useState<SavedCustomModifier[]>(() =>
+    loadCustomModifierLibrary(),
+  );
+  useEffect(() => {
+    if (formOpen) setCustoms(loadCustomModifierLibrary());
+  }, [formOpen]);
   const [kindId, setKindId] = useState<string>(descriptors[0]?.id ?? '');
   const [rawValue, setRawValue] = useState('');
 
   const descriptor = descriptors.find((d) => d.id === kindId);
+  const customDescriptor = customs.find((c) => c.id === kindId);
+  const isCustomKind = customDescriptor !== undefined;
+
   const parsedValue = descriptor ? parseFormValue(descriptor, rawValue) : undefined;
-  const isValid =
+  const isValid = isCustomKind
-    descriptor !== undefined &&
+    ? true // custom kinds carry no per-instance value; descriptor is applied as-is.
+    : descriptor !== undefined &&
       parsedValue !== undefined &&
       descriptor.valueSchema.safeParse(parsedValue).success;
 
@@ -81,13 +98,27 @@ export function PerTypePanel({ modifiers, clipboard, onClipboardChange, onPaste,
   }
 
   function handleSave() {
-    if (!isValid || !descriptor) return;
+    if (!isValid) return;
+    if (isCustomKind && customDescriptor) {
+      // Custom modifiers don't take a per-instance value; the descriptor's
+      // primitive list is the entire payload. Pass `null` so the apply-time
+      // dispatcher (T22) sees a present-but-meaningless value.
+      onAdd({
+        kind: customDescriptor.id,
+        pieceType,
+        color,
+        value: null,
+      });
+    } else if (descriptor) {
       onAdd({
         kind: descriptor.id,
         pieceType,
         color,
         value: parsedValue,
       });
+    } else {
+      return;
+    }
     setFormOpen(false);
     resetForm();
   }
@@ -229,16 +260,32 @@ export function PerTypePanel({ modifiers, clipboard, onClipboardChange, onPaste,
                 }}
                 className="w-full px-2 py-1.5 text-sm border border-neutral-300 rounded focus:outline-none focus:ring-2 focus:ring-neutral-900"
               >
+                <optgroup label="Built-in">
                   {descriptors.map((d) => (
                     <option key={d.id} value={d.id}>
                       {d.label}
                     </option>
                   ))}
+                </optgroup>
+                {customs.length > 0 && (
+                  <optgroup label="Custom (from library)">
+                    {customs.map((c) => (
+                      <option
+                        key={c.id}
+                        value={c.id}
+                        data-testid={`kind-custom-${c.id}`}
+                      >
+                        {c.descriptor.name}
+                      </option>
+                    ))}
+                  </optgroup>
+                )}
               </select>
             </div>
 
-            {/* Value input — rendered by uiForm type */}
+            {/* Value input — rendered by uiForm type. Custom kinds skip
-            {descriptor !== undefined && (
+                this entirely; the descriptor's primitive list is the payload. */}
+            {!isCustomKind && descriptor !== undefined && (
               <div>
                 <label className="block text-xs font-semibold text-neutral-500 uppercase tracking-wide mb-1">
                   Value
@@ -250,6 +297,17 @@ export function PerTypePanel({ modifiers, clipboard, onClipboardChange, onPaste,
                 />
               </div>
             )}
+            {isCustomKind && customDescriptor !== undefined && (
+              <div
+                data-testid="custom-modifier-summary"
+                className="px-3 py-2 text-xs text-neutral-600 bg-neutral-50 border border-neutral-200 rounded"
+              >
+                Custom modifier — {customDescriptor.descriptor.primitives.length}{' '}
+                primitive
+                {customDescriptor.descriptor.primitives.length !== 1 ? 's' : ''}.
+                Edit it in the Custom Modifier editor.
+              </div>
+            )}
 
             {/* Actions */}
             <div className="flex gap-2 pt-1">