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.

include/grid.h

@@ -12,6 +12,7 @@ void grid_init(grid_t *g, int cols, int rows);
 void grid_free(grid_t *g);
 void grid_resize(grid_t *g, int new_cols, int new_rows);
 void grid_clear(grid_t *g);
+void grid_copy(grid_t *dst, const grid_t *src);
 int grid_get(const grid_t *g, int c, int r);
 void grid_set(grid_t *g, int c, int r, int val);