feat: default timelines to a shared ambient clock, overridable per timeline

[johngeorgewright]

Why

Threading an explicit { clock } through every Timeline.create proved to be the wrong default: in practice a source, the code under test, and an expectation always need the same clock, and forgetting to share it produced a class of confusing bugs (timer races, deadlocks). The fake-timers clock is already a process-global singleton anyway, so injection was fighting reality.

What

• Ambient default clock. getDefaultClock() returns a shared clock; Timeline uses it unless given an explicit { clock } (the override is unchanged). setDefaultClock(clock) swaps it and returns a restore handle to pair with test teardown, so the ambient clock doesn't leak between tests.
• Per-frame timer onPass. A passed Tn now advances one frame at a time (like dash) instead of jumping its whole duration in one synchronous step. This lets two timelines sharing a clock interleave in lock-step — fixing the "source races ahead of the expectation's barrier" failure on direct source → expectation timing.

Tests

tsc (src + test projects) clean, build clean, bun test → 13 pass, including new coverage: timelines share the ambient clock by default, an explicit clock overrides it, and setDefaultClock swaps/restores. Existing streamBridge end-to-end proof still passes.

Note

Behavioural change: the default clock is now shared rather than fresh-per-Timeline. The { clock } override restores per-timeline isolation where needed. Tests that assert absolute clock.now should reset the ambient clock in a beforeEach (setDefaultClock(new Clock())), as this PR's own tests now do.

🤖 Generated with Claude Code