Module clock 

SharedClock - the single playback time source (decision 2).

Both the Player (worker thread) and the UI (main thread) read the SAME clock, so highlight and audio physically can’t drift. The Player writes position; the UI’s ~15 Hz timer reads SharedClock::position_s and highlights the WordMark whose time_s <= position.

Cross-thread via a relaxed AtomicU64 (nanoseconds) - cheap and single-producer (Player) / single-consumer (UI timer).

Structs

SharedClock

Shared playback position in nanoseconds (0 = start of playback).

Functions

highlight_index

Pick the index of the last WordMark whose time_s is <= position_s (the currently-highlighted word). Returns None before the first word.