Open the add menu with a right-click in the asset panel (#244)

New assets could only be added through the + button. A right-click
on a category heading or on the empty space below the rows now opens
the same menu at the pointer.

- ListView gets the callback onListMenu. It fires on a right-click
  on empty space or on a heading. Only the asset panel has headings,
  so other lists are not affected.
- AssetPanel opens its + menu from there.
- A right-click on a heading does not fold or unfold the category.
  A right-click on empty space clears the selection like a left-click.
- A right-click on an asset still opens the menu of that asset.
- The empty list now says "No assets - press + or right-click".

Tests:
- ./check.sh: 178 files, 2573 tests, CHECK EXIT 0
- e2e 1-4: ALL PASSED, 134 checks
- 3 new checks in e2e 04 with real right-clicks: menu on the
  "Scenes" heading (21 types, heading stays folded), menu on empty
  space, and "Behavior" from that menu creates one, undone again.
- Counter-probe: without the menu both right-click checks fail.

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

tools/e2e/04-assets.mjs

@@ -32,6 +32,8 @@ export async function assets(t) {
   await shot('11-context-menu');
   await press('Escape');
 
+  await addMenuOnRightClick(t);
+
   // 4b. Drag the Crate asset from the panel into the scene view. Driven step by step rather than
   //     with `drag()`, because the preview only exists while the button is still down.
   const from = await json("__gm.assetRow('crate')");
@@ -443,3 +445,50 @@ export async function assets(t) {
     JSON.stringify({ first, typeOfFirst, rowAt, iconAt, ink, typeWords }),
   );
 }
+
+/**
+ * 4a-ii. Where no menu of its own opens – on a group heading or on the empty space below the rows –
+ * a right-click brings up the + menu (#244). The heading stays folded as it was.
+ */
+async function addMenuOnRightClick(t) {
+  const { check, json, shot, click, clickText, press, undo, showTab } = t;
+  const addRows = (rows) =>
+    Array.isArray(rows) && rows.filter(Boolean).length === 21 && rows.includes('Sprite');
+  const heading = await json("__gm.assetRowShown('group:scene')");
+  const sceneFolded = async () =>
+    !(await json('__gm.assetGroups()')).find((g) => g.type === 'scene').expanded;
+  const foldedBefore = await sceneFolded();
+  await click(heading.x, heading.y, 'right');
+  const onHeading = await json('__gm.menuRows()');
+  check(
+    'a right-click on a group heading opens the + menu and leaves the group as it was',
+    addRows(onHeading) && (await sceneFolded()) === foldedBefore,
+    JSON.stringify({ onHeading, foldedBefore }),
+  );
+  await press('Escape');
+  const groups = await json('__gm.assetGroups()');
+  const lastGroup = groups.at(-1);
+  const lastShown = await json(
+    `__gm.assetRowShown(${JSON.stringify(lastGroup.expanded ? lastGroup.ids.at(-1) : `group:${lastGroup.type}`)})`,
+  );
+  const panel = await json('__gm.assetsRect()');
+  const empty = { x: panel.x + panel.w / 2, y: panel.y + panel.h - 12 };
+  await click(empty.x, empty.y, 'right');
+  const onEmpty = await json('__gm.menuRows()');
+  check(
+    'a right-click on the empty space below the rows opens the + menu',
+    empty.y > lastShown.y + 20 && addRows(onEmpty),
+    JSON.stringify({ empty, lastShown, onEmpty }),
+  );
+  await shot('11c-add-menu-on-right-click');
+  const before = (await json('__gm.state.project.assets')).length;
+  await clickText('Behavior');
+  const fromMenu = await json('__gm.state.project.assets.at(-1)');
+  check(
+    'an entry of that menu creates the asset like the + button does',
+    (await json('__gm.state.project.assets')).length === before + 1 && fromMenu.type === 'behavior',
+    JSON.stringify(fromMenu && { type: fromMenu.type, name: fromMenu.name }),
+  );
+  await undo();
+  await showTab('scene');
+}