chore(v→zig): delete legacy V-lang API shim (vext/api/v/vext.v) (#27)

Per estate-wide owner directive (2026-05-28): Zig is the default for
APIs/FFIs; Idris2 owns ABIs. The deleted file was a thin V wrapper around
the existing Zig FFI at vext/ffi/zig/. With V removed from the estate,
the Zig FFI is the canonical API surface — no replacement needed.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath