Give the graph a loop, and build Tetris with it

A Tetris example project needs to draw 200 board cells and scan rows
for a full line. Neither is expressible without iteration, so the node
system gets a Repeat node plus the missing text side: Substring, Find
text and To number. The interpreter drops its data cache once per
round -- otherwise every round would recompute with the first round's
values.

Object by name takes its name as an input now, not just as a setting;
addressing "c" + index is impossible otherwise. An unconnected input
reads the stored setting, so saved projects are untouched.

The board itself is a 200-character text, "." for empty. That makes
both operations of Tetris a single expression instead of a loop, and
each frame attempts exactly one move -- rotate beats sideways beats
fall -- so the fit check exists once in the whole graph.

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

src/core/graph/nodes/data/Substring.ts

@@ -0,0 +1,21 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { asNumber, asString } from '../../values';
+import { num, str, withDefault } from '../shared';
+
+export const Substring: NodeDef = {
+  type: 'Substring',
+  label: 'Substring',
+  category: 'data',
+  description:
+    'A part of a text: "count" characters starting at "start" (the first is 0). ' +
+    'A negative count reaches to the end.',
+  inputs: [str('text'), num('start'), withDefault(num('count'), -1)],
+  outputs: [str('text')],
+  params: [],
+  run: (_ctx, inputs) => {
+    const text = asString(inputs.text);
+    const start = Math.max(0, Math.floor(asNumber(inputs.start)));
+    const count = Math.floor(asNumber(inputs.count));
+    return { outputs: { text: count < 0 ? text.slice(start) : text.slice(start, start + count) } };
+  },
+};