A new node takes the first of two values that is not empty (#205)

A graph that reads an optional value needed a chain of Equals and
Branch just to fall back on a default. The new node does that in one
step: it passes a through unless a is empty, in which case b comes
out. Both ports are any, so it works for numbers, text, lists,
structs and objects alike.

- `First not empty` sits in the data category with two any inputs
  `a` and `b` and one any output `value`. Are both empty, b comes
  out, so the second input doubles as the default.
- `isEmptyValue` in values.ts holds the rule as a pure function:
  nothing connected, 0, an empty text, false, an empty list, and a
  struct whose fields are all empty.
- The struct case is what covers a position: behavior graphs have no
  vector type of their own, so (0, 0) arrives as a struct. `{x: 0,
  y: 2}` therefore counts as set, `{x: 0, y: 0}` as empty.

Findings:

- A real 0 or false on a is passed over. That follows from counting
  0 as empty and is written down in the issue.
- The output stays any. Ports that inherit their type from an input
  do not exist in the graph; `Item at` has the same shape.

Tests:

- ./check.sh: 142 files, 2104 tests, exit 0 (was 2094).
- 10 new catalog cases: a set, a unconnected, 0, "", false, an empty
  list, a position (0, 0), a struct with one field set, an object,
  and both empty.
- Counter-check: with structs never empty and 0 a real value exactly
  those two cases fail, the rest stays green. Originals restored and
  compared with cmp, then rebuilt.
- No e2e check: single data nodes have no e2e coverage here, the
  palette fills itself from BUILT_IN_NODES.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/values.ts

@@ -91,6 +91,18 @@ export function asString(value: Value | undefined): string {
   return textOf(isValueList(value) ? storedList(value) : value);
 }
 
+/**
+ * Whether a value is the empty one of its kind: no object, 0, '', false, an empty list, or a struct
+ * whose fields are all empty – which is how a position (0, 0) arrives.
+ */
+export function isEmptyValue(value: Value | undefined): boolean {
+  if (value === null || value === undefined) return true;
+  if (isEntity(value)) return false;
+  if (isValueList(value)) return value.length === 0;
+  if (isStructValue(value)) return Object.values(value).every(isEmptyValue);
+  return value === 0 || value === '' || value === false;
+}
+
 export function asEntity(value: Value | undefined): EntityHandle | null {
   return isEntity(value) ? value : null;
 }