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>

refimpl/go.mod

@@ -0,0 +1,7 @@
+module github.com/nennemann/ect-refimpl
+
+go 1.22
+
+require github.com/go-jose/go-jose/v4 v4.0.2
+
+require golang.org/x/crypto v0.28.0 // indirect