Pick an existing collider or rigidbody from the object menu (#222)

"Add collider" and "Add rigidbody" could only create a fresh asset.
Reusing one meant creating a copy by hand, and a new box collider
never matched the sprite it was added to.

- "Add collider…" and "Add rigidbody…" open a second menu at the same
  spot: a search field, "Create new …", a separator, then every asset
  of that kind sorted by name. Enter takes the first match.
- An existing asset is placed as a shared child, not copied. An asset
  the object already carries is greyed out.
- The rule for when the entries are available lives in one place in
  the collider commands. An object without a resolvable asset is
  greyed out again, as with the old canAddCollider.
- "Fit to sprite" in the menu of a placed collider sizes it to its
  sprite and centres it (position 0,0, z kept) as one undo step.
- Fit writes overrides of that placement, not the asset, because a
  collider can now sit on several sprites. A value equal to the asset
  value clears the override instead.
- Fit removes the collider's own scale, swaps width and height near
  90 degrees and gives a circle the radius that fits (min/2). It is
  greyed out for polygon and frame outlines and off a sprite.
- Menu has no submenus, so a second menu opens instead, similar to
  "Add Component" in Unity. That costs one click more than before.

Tests:
- ./check.sh: 152 files, 2234 tests, CHECK EXIT 0 (before 151/2228).
- 6 new unit tests in tests/core/collider-parts.test.ts.
- 3 new e2e checks in 08a: submenu with search and greying, an
  existing collider via search and Enter, Fit on oc2.
- 09, 40 and 41 follow the new labels.
- e2e 1-16,39-41 on the final build: ALL PASSED, 596 checks.
- Counter-checks: no greying, existing asset ignored and fit without
  centring each fail at their own check. Breaking the override rule
  fails 2 unit tests.
- CI confirmed 1345 checks at 2cf7202 (task 265). New total 1348,
  calculated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/objectMenu.ts

@@ -1,3 +1,4 @@
+import { colliderFit, FitColliderCommand } from '../core/commands/colliderCommands';
 import { UnpackContainerCommand } from '../core/commands/containerCommands';
 import { SetParentCommand } from '../core/commands/sceneObjectCommands';
 import { findObject } from '../core/model/hierarchy';
@@ -64,15 +65,24 @@ export function openObjectMenu(
         )
       : []),
     {
-      label: 'Add collider',
+      label: 'Add collider…',
       disabled: !object || many || !canAddCollider(state.project, source, object),
-      onSelect: () => addCollider(state, holder.id, id),
+      onSelect: () => openPartMenu(state, 'collider', holder, id, x, y),
     },
     {
-      label: 'Add rigidbody',
+      label: 'Add rigidbody…',
       disabled: !object || many || !canAddRigidbody(state.project, source, object),
-      onSelect: () => addRigidbody(state, holder.id, id),
+      onSelect: () => openPartMenu(state, 'rigidbody', holder, id, x, y),
     },
+    ...(object && asset?.type === 'collider'
+      ? [
+          {
+            label: 'Fit to sprite',
+            disabled: many || typeof colliderFit(state.project, source, object) === 'string',
+            onSelect: () => state.execute(new FitColliderCommand(holder.id, id)),
+          },
+        ]
+      : []),
     {
       label: many ? `Create container from ${selected.length} objects` : 'Create container',
       disabled: selected.some((oid) => isTemplateContent(source, oid)),
@@ -102,6 +112,39 @@ export function openObjectMenu(
   return menu;
 }
 
+/**
+ * Which collider or rigidbody to add (#222): a new one, or any the project has, found by typing. One
+ * the object wears already is greyed out – a second copy of it would only double up.
+ */
+function openPartMenu(
+  state: EditorState,
+  type: 'collider' | 'rigidbody',
+  holder: ObjectHolder,
+  id: string,
+  x: number,
+  y: number,
+): Menu {
+  const add = type === 'collider' ? addCollider : addRigidbody;
+  const worn = new Set(holder.objects.filter((o) => o.parentId === id).map((o) => o.assetId));
+  const existing = state.project.assets
+    .filter((a) => a.type === type)
+    .sort((a, b) => a.name.localeCompare(b.name));
+  const menu = new Menu(
+    [
+      { label: `Create new ${type}`, onSelect: () => add(state, holder.id, id) },
+      { separator: true },
+      ...existing.map((a) => ({
+        label: a.name,
+        disabled: worn.has(a.id),
+        onSelect: () => add(state, holder.id, id, a.id),
+      })),
+    ],
+    { searchable: true },
+  );
+  menu.open(x, y);
+  return menu;
+}
+
 /**
  * "Open texture", "Open shader", … – one entry per asset the values point at (#221), skipping kinds
  * that have no tab to open. The asset panel offers the same for an asset's own values.