Use a single micros() call in WarnIfComponentBlockingGuard::finish()
for both runtime stats recording and blocking detection. Previously
finish() called millis() for blocking detection and a separate
non-inlined record_runtime_stats_() that called micros() again.
Changes:
- Inline record_runtime_stats_() to avoid function call overhead
- Derive curr_time from micros()/1000 instead of separate millis() call
- Override started_ from micros()/1000 in constructor so both timestamps
use the same clock source (fixes unsigned underflow on platforms where
millis() and micros()/1000 can diverge)
- Extract check_blocking_() as inline helper for readability