After Ticker.stop(), elapsed resets to zero on the next start(). _now was
left frozen at the old elapsed, so effects added after a drain captured a
stale start time and their progress() clamped to 0 forever — ticker never
stopped, second batch broken. Fix: reset _now = Duration.zero on drain.
Adds @visibleForTesting getters and a regression test that catches the
stale value directly.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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