ports/stm: note atmel-samd freq bias near TIM6 round-to-nearest

Sweep audits across boards showed atmel-samd has a constant -3.4 cent
bias on every tone, consistent with truncating its TIM period. Flag
that next to our round-to-nearest so the fix is portable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>