feat(contract): D-V3-W6a adoption-scan counting logic — ClassidForm + count_adoption

Mirrors classid_canon_compat's decision procedure exactly (zero bit
math): CanonHigh vs the three documented legacy shapes (zero-prefix,
pre-flip V3-marker-high, render-prefix-high) per routing.md §5.
Ambiguous variant reserved (#[non_exhaustive]) for the documented
0x1000-domain collision bound; never constructed today. Tests via
sanctioned composers only (F12). 10/10 new tests; full contract suite
771 green. Baseline finding: no real classid-keyed Lance corpora in
this container (toy fixture only) — t0 baseline runs where data lives
once the CLI wrapper lands.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MLBnPuScZy6w9di2QEjsXM

Author: claude