Put one item into a list, a struct staying whole, with Add item (#250)

A list could only grow through Join lists, and that takes its b apart:
a struct becomes one item per field and a list is joined. A list of
structs such as loot could not grow at all.

- Add item returns a new list with value as exactly one more item. A
  struct or a list goes in whole. The list that came in is left alone,
  like with Set item.
- index -1 or unset appends. Otherwise the item goes in before that
  index. An index outside the list appends, so the value is not lost.
- The rule that an item takes the kind of its slot moved out of Set item
  into fitToSlot. Both nodes use it, Set item behaves as before.
- No e2e check and no browser look. The node is covered by unit tests.

Tests:
- ./check.sh: 180 files, 2634 tests, CHECK EXIT 0
- 7 catalog cases, 4 cases in list-of-structs.test.ts, and index in the
  whole-number inputs of texture-kind.test.ts
- Counter-check: taking the value apart like Join lists fails 3 tests
- This commit alone: tsc and the three touched unit files pass
  (337 tests)

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

tests/core/node-catalog.test.ts

@@ -728,6 +728,35 @@ const CASES: Record<string, Case[]> = {
       outputs: { result: [1, 2] },
     },
   ],
+  AddItem: [
+    { name: 'appends by default', inputs: { list: [1, 2], value: 3 }, outputs: { result: [1, 2, 3] } },
+    {
+      name: 'inserts before index',
+      inputs: { list: [1, 2, 3], value: 9, index: 1 },
+      outputs: { result: [1, 9, 2, 3] },
+    },
+    {
+      name: 'index equal to the length appends',
+      inputs: { list: [1, 2], value: 9, index: 2 },
+      outputs: { result: [1, 2, 9] },
+    },
+    {
+      name: 'an index outside the list appends instead of dropping the value',
+      inputs: { list: [1, 2], value: 9, index: 7 },
+      outputs: { result: [1, 2, 9] },
+    },
+    {
+      name: 'leaves the list it was given untouched – a graph may hold it elsewhere',
+      inputs: { list: Object.freeze([1, 2]), value: 3 },
+      outputs: { result: [1, 2, 3] },
+    },
+    {
+      name: 'a list goes in as one item, not joined',
+      inputs: { list: [[1]], value: [2, 3] },
+      outputs: { result: [[1], [2, 3]] },
+    },
+    { name: 'an unset list starts empty', inputs: { value: 5 }, outputs: { result: [5] } },
+  ],
   SubList: [
     {
       name: 'takes count items from start',