docs: rewrite VQL execution modes — two-tier model (VQL + VQL-UT)

VQL-DT is not a separate product; it is the proof-carrying execution
mode within VQL, now formally folded into VQL-UT levels L9-L10.

Replaces the old three-path model (VQL/VQL-DT/VQL-UT) with a clean
two-tier model: VQL (the language) + VQL-UT (the progressive safety
pipeline). Slipstream queries exit early; PROOF clauses activate L9-L10.

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

Author: hyperpolymath