Group three co-assigned fields (_start_frame_id, _start_frame_arrival_ns,
_start_wall_clock) into a single _StartFrameInfo dataclass. This makes
the "always set together" invariant structural rather than implicit, and
fixes the incorrect str | None annotation on _start_frame_id (Frame.id
is int).
Add pyrightconfig.json with basic type checking for zero-error modules
(clocks, metrics, transcriptions, frames) and enforce via CI. The
include list will expand as modules are fixed.
