chore(v-sweep): delete V-lang VeriSimDB client source

Removes 9 V source files and v.mod from connectors/clients/vlang/. The
Rust client at connectors/clients/rust/ provides identical coverage
(client, search, drift, provenance, federation, octad, vcl, error,
types modules). MIGRATION.adoc (added previous commit) documents the
module-by-module mapping.

V-lang banned 2026-04-10. Retaining vlang/ dir with only MIGRATION.adoc
as a breadcrumb for anyone arriving at the old path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath