Tell On timer how late its frame is, so time minus late is exact (#224)

A timer can only fire on a frame. It fires on the first frame past each
due time, so Time.time at that moment is the due time plus up to one
frame. On a display that is not exactly 60 Hz this delay creeps up by
a millisecond every few seconds and falls back to zero once it reaches
a whole frame. The issue saw 82.009 after 82 ticks. The timer itself
does not drift, it keeps the remainder when it fires.

- On timer gets a number output late. It is what is left of the
  timer's clock after taking one interval, so time - late is exactly
  the due time.
- When one long frame catches up several ticks, each gets its own
  late. The first one was due longest ago and is the latest.

Tests:
- New unit test in behavior-events: late 0.125 after a frame past
  1.0 s, 0 on the dot, 1.5 and 0.5 for two ticks caught up in one
  frame.
- Counter-check: late taken before the interval is subtracted fails
  exactly that test. Original restored with cmp.
- ./check.sh: 156 files, 2300 tests, CHECK EXIT 0 (was 2299).
- No e2e change. The e2e modules only wire the exec of On timer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/behavior-events.test.ts

@@ -262,6 +262,24 @@ describe('On timer', () => {
     expect(world.log).toEqual(['tick', 'tick', 'tick', 'tick']);
   });
 
+  it('late is how far the firing frame is past the due time, so time minus late is exact', () => {
+    const f = scene();
+    const edit = graphEditor(f, f.behavior);
+    const timer = edit.addNode('OnTimer', { interval: 1 });
+    const log = edit.addNode('Log', {});
+    edit.connect(timer, 'exec', log, 'exec');
+    edit.connect(timer, 'late', log, 'message');
+    const { world, frame } = startWorld(f, { behaviors: true });
+
+    frame(0.75);
+    frame(0.375); // 1.125 s: due at 1.0
+    expect(world.log).toEqual(['0.125']);
+    frame(0.875); // 2.0 s: due exactly now
+    expect(world.log).toEqual(['0.125', '0']);
+    frame(2.5); // 4.5 s: catches up 3.0 and 4.0, the first one longer ago
+    expect(world.log).toEqual(['0.125', '0', '1.5', '0.5']);
+  });
+
   it('two timers keep separate clocks', () => {
     const f = scene();
     logOn(f, 'OnTimer', { interval: 1 }, 'slow');