feat: implement QTT quantity checker and wire into type checker

Rewrote quantity.ml with correct semiring (0/1/ω), fixed critical bug
where linear variables with zero uses passed silently. Wired into
typecheck.ml as a post-type-checking pass. Error codes E0300-E0303.
Clean build with dune.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath