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/go-lang/ect/jti_cache_test.go

@@ -0,0 +1,56 @@
+package ect
+
+import (
+	"testing"
+	"time"
+)
+
+func TestJTICache_SeenAndAdd(t *testing.T) {
+	cache := NewJTICache(10, 1*time.Second)
+	if cache.Seen("jti-1") {
+		t.Error("unseen jti should not be seen")
+	}
+	cache.Add("jti-1")
+	if !cache.Seen("jti-1") {
+		t.Error("added jti should be seen")
+	}
+	if cache.Seen("jti-2") {
+		t.Error("other jti should not be seen")
+	}
+	cache.Add("jti-2")
+	if !cache.Seen("jti-2") {
+		t.Error("second jti should be seen")
+	}
+}
+
+func TestJTICache_Expiry(t *testing.T) {
+	cache := NewJTICache(10, 50*time.Millisecond)
+	cache.Add("jti-1")
+	if !cache.Seen("jti-1") {
+		t.Fatal("added jti should be seen")
+	}
+	time.Sleep(60 * time.Millisecond)
+	if cache.Seen("jti-1") {
+		t.Error("expired jti should not be seen")
+	}
+}
+
+func TestJTICache_MaxSizeEviction(t *testing.T) {
+	cache := NewJTICache(2, 5*time.Second)
+	cache.Add("jti-1")
+	cache.Add("jti-2")
+	cache.Add("jti-3") // evicts one
+	// At least one of jti-1, jti-2 may be evicted; jti-3 must be present
+	if !cache.Seen("jti-3") {
+		t.Error("newest jti should be present")
+	}
+}
+
+func TestJTICache_AddWhenAlreadyPresent(t *testing.T) {
+	cache := NewJTICache(2, 5*time.Second)
+	cache.Add("jti-1")
+	cache.Add("jti-1") // refresh same jti; should not evict
+	if !cache.Seen("jti-1") {
+		t.Error("jti-1 should still be present")
+	}
+}