chore(v→zig): delete legacy V-lang API shim (#119)

## Summary

Estate-wide V-lang→Zig remediation per the 2026-05-28 owner directive.
Deletes `api/v/echidna.v` (thin V-lang wrapper around the existing Zig
FFI at `ffi/zig/`). Zig FFI is the canonical API surface.

**Note:** this repo retains Coq `.v` files at `examples/` and
`tests/live_goals/` (theorem-prover input fixtures). Those are not
V-lang and are exempted in the new hypatia rule.

## Test plan

- [ ] Hypatia rule \`cicd_rules/vlang_detected\` does not flag this repo
post-merge
- [ ] Coq fixtures at `examples/` and `tests/live_goals/` remain intact

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath