fix(live): avoid hangs at fractional transcript grouping deadlines (#3970)

Valid fractional Live transcript grouping timeouts can hang on the next
transcript event. For example, after a fragment ends at 200 ms, a
timeout of 0.1 ms is scheduled for 200.1; subtracting 200 later rounds
to less than 0.1, so the scheduler never consumes that deadline.

Compare absolute times against the same deadlines that the scheduler
creates. This preserves the existing API and options and adds bounded
sync/async regressions for assistant silence, speaker separation, and
isolated backchannels.

Validation:
- All six regressions safely reproduced the hang before the fix.
- 224/224 Live tests passed on both Pydantic v1 and v2 after the fix.
- Full Ruff, mypy and Pyright passed.
- No schema, generated SDK, dependency or API changes.