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>

tools/lib/harness.mjs

@@ -32,7 +32,9 @@ const KEYS = {
   Escape: { key: 'Escape', code: 'Escape', windowsVirtualKeyCode: 27 },
   Space: { key: ' ', code: 'Space', windowsVirtualKeyCode: 32 },
   ArrowLeft: { key: 'ArrowLeft', code: 'ArrowLeft', windowsVirtualKeyCode: 37 },
+  ArrowUp: { key: 'ArrowUp', code: 'ArrowUp', windowsVirtualKeyCode: 38 },
   ArrowRight: { key: 'ArrowRight', code: 'ArrowRight', windowsVirtualKeyCode: 39 },
+  ArrowDown: { key: 'ArrowDown', code: 'ArrowDown', windowsVirtualKeyCode: 40 },
   Delete: { key: 'Delete', code: 'Delete', windowsVirtualKeyCode: 46 },
   a: { key: 'a', code: 'KeyA', windowsVirtualKeyCode: 65 },
   g: { key: 'g', code: 'KeyG', windowsVirtualKeyCode: 71 },