Restructure refimpl into go-lang and python subdirectories

Move Go reference implementation to refimpl/go-lang/ and add new
Python reference implementation in refimpl/python/. Update build.sh
with renamed draft and simplified tool paths.

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

refimpl/python/tests/test_jti_cache.py

@@ -0,0 +1,40 @@
+"""Tests for JTI replay cache."""
+
+import time
+
+import pytest
+
+from ect import new_jti_cache
+
+
+def test_jti_cache_seen_and_add():
+    cache = new_jti_cache(10, 60)
+    assert cache.seen("jti-1") is False
+    cache.add("jti-1")
+    assert cache.seen("jti-1") is True
+    assert cache.seen("jti-2") is False
+    cache.add("jti-2")
+    assert cache.seen("jti-2") is True
+
+
+def test_jti_cache_expiry():
+    cache = new_jti_cache(10, 1)  # 1 second TTL
+    cache.add("jti-1")
+    assert cache.seen("jti-1") is True
+    time.sleep(1.1)
+    assert cache.seen("jti-1") is False
+
+
+def test_jti_cache_max_size_eviction():
+    cache = new_jti_cache(2, 60)
+    cache.add("jti-1")
+    cache.add("jti-2")
+    cache.add("jti-3")
+    assert cache.seen("jti-3") is True
+
+
+def test_jti_cache_add_when_already_present():
+    cache = new_jti_cache(2, 60)
+    cache.add("jti-1")
+    cache.add("jti-1")
+    assert cache.seen("jti-1") is True