perf(render): redraw on demand and only changed cells

Rendering used to clear the whole canvas and redraw every cell on
every frame callback (~60 fps) even though the simulation ticks at
20 tps and starts paused, keeping CPU usage at ~85% while idle.

Split simulation timing out of redraw() into the event loop and
render only when displayed content can have changed (tick, painting,
pattern placement, preview movement, grid-line toggle, resize);
idle frames commit only a new frame callback. Repaint incrementally
by diffing the grid against a rendered-state snapshot per row and
redrawing just the changed cells, with damage limited to the
repainted bounding box.

Idle CPU drops to ~1% of one core; running frames render at tick
rate (~20 fps) at microsecond cost. Adds grid_copy() snapshot
helper with tests and archives the optimize-render-on-demand change
with the new on-demand-rendering spec.

tests/test_grid.c

@@ -19,6 +19,32 @@ int main(void) {
     grid_clear(&g);
     assert(grid_get(&g, 0, 0) == 0);
 
+    /* grid_copy: same-size round trip */
+    grid_t snap;
+    grid_init(&snap, 1, 1);
+    grid_set(&g, 2, 1, 1);
+    grid_copy(&snap, &g);
+    assert(snap.cols == 4 && snap.rows == 4);
+    assert(grid_get(&snap, 2, 1) == 1);
+    assert(grid_get(&snap, 0, 0) == 0);
+
+    /* grid_copy: grow preserves cells, new area is dead */
+    grid_set(&g, 3, 3, 1);
+    grid_resize(&g, 6, 6);
+    grid_copy(&snap, &g);
+    assert(snap.cols == 6 && snap.rows == 6);
+    assert(grid_get(&snap, 2, 1) == 1);
+    assert(grid_get(&snap, 3, 3) == 1);
+    assert(grid_get(&snap, 5, 5) == 0);
+
+    /* grid_copy: shrink truncates out-of-bounds cells */
+    grid_resize(&g, 2, 2);
+    grid_copy(&snap, &g);
+    assert(snap.cols == 2 && snap.rows == 2);
+    assert(grid_get(&snap, 2, 1) == 0);
+    assert(grid_get(&snap, 1, 1) == 0);
+
+    grid_free(&snap);
     grid_free(&g);
     printf("grid tests passed\n");
     return 0;