Add WIMSE ECT reference implementation (Go)

- ect library: create, verify, DAG validation, ledger interface
- In-memory ledger and ECTStore for full ledger mode
- Test vectors and unit tests; two-agent demo (cmd/demo)
- README: document refimpl scope and usage

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -21,6 +21,7 @@ Target environments include medtech (FDA audit trails), finance (transaction rec
 |------|-------------|
 | `draft-nennemann-wimse-execution-context-00.md` | The Internet-Draft in kramdown-rfc markdown format |
 | `master-prompt.md` | Design rationale, iteration plan, and reference material |
+| `refimpl/` | **Reference implementation** (Go): ECT create/verify, DAG validation, in-memory ledger, and a two-agent demo. See `refimpl/README.md`. |
 
 ## Building the draft