Extracts the YYYY-MM-DD_<tracking> (<status>) - <title> grammar into a
reusable parser in the zddc package, and exposes a tracking-type
predicate for -TRN- / -SUB- (case-fold). The transmittal-folder regex
was previously only inside archive/index.go where it captured just the
date; the new ParseTransmittalFolder also returns tracking, status, and
title so handlers can recognise transmittal envelopes for upcoming
staging↔working mirror logic.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
