fix(persist,channel): 同类定宽时间戳修复 — checkpoint 恢复与渠道会话列表排序

- persist: formatTime 定宽；loadLatestSQL 补 rowid 兜底；New 时归一化历史行（避免 LoadLatest 挑到更旧检查点）
- channel/session: 写入定宽；listSessionSQL 补 rowid 兜底；核实无 INSERT 吃 DDL 默认值，故只加注释不改默认值
- 新增回归测试并做变异验证（改回 RFC3339Nano 即 FAIL）

internal/channel/session/manager_test.go

@@ -318,3 +318,98 @@ func TestNewRejectsNilDB(t *testing.T) {
 		t.Fatalf("New(nil db) error = %v, want ErrNilDB", err)
 	}
 }
+
+// insertSessionAt writes a channel_sessions row directly with a caller-supplied
+// raw created_at/updated_at, standing in for a mapping written by past code.
+func insertSessionAt(t *testing.T, handle *sql.DB, cid, at string) {
+	t.Helper()
+	if _, err := handle.ExecContext(context.Background(),
+		`INSERT INTO channel_sessions (platform, channel_id, session_id, actor_role, created_at, updated_at)
+		 VALUES ('qq', ?, ?, 'owner', ?, ?)`,
+		cid, "sess-"+cid, at, at); err != nil {
+		t.Fatalf("insert %s: %v", cid, err)
+	}
+}
+
+// TestListNewestFirstDeterministic covers the ordering contract: three mappings
+// written within the same coarse clock tick share one created_at, so only the
+// rowid tiebreak can define their order. List is newest first, so the row
+// inserted last (highest rowid) must come first. The cache is deliberately empty
+// so List reads the persisted order instead of cached entries.
+func TestListNewestFirstDeterministic(t *testing.T) {
+	t.Parallel()
+
+	m, handle := newTestManager(t)
+	ctx := context.Background()
+
+	const at = "2026-09-07T03:20:05.830000000Z"
+	for _, cid := range []string{"direct-a", "direct-b", "direct-c"} {
+		insertSessionAt(t, handle, cid, at)
+	}
+
+	got, err := m.List(ctx)
+	if err != nil {
+		t.Fatalf("List = %v", err)
+	}
+	want := []string{"direct-c", "direct-b", "direct-a"}
+	if len(got) != len(want) {
+		t.Fatalf("List = %d sessions, want %d", len(got), len(want))
+	}
+	for i, w := range want {
+		if got[i].ChannelID != w {
+			t.Errorf("got[%d].ChannelID = %q, want %q (rowid DESC tiebreak)", i, got[i].ChannelID, w)
+		}
+	}
+}
+
+// TestListNewestFirstByCreatedAt checks the created_at DESC half: the mapping
+// with the latest fixed-width timestamp is listed first even though it was
+// inserted first and therefore has the lowest rowid.
+func TestListNewestFirstByCreatedAt(t *testing.T) {
+	t.Parallel()
+
+	m, handle := newTestManager(t)
+	ctx := context.Background()
+
+	insertSessionAt(t, handle, "direct-newest", "2026-09-07T03:20:05.830978900Z")
+	insertSessionAt(t, handle, "direct-oldest", "2026-09-07T03:20:05.830000000Z")
+
+	got, err := m.List(ctx)
+	if err != nil {
+		t.Fatalf("List = %v", err)
+	}
+	if len(got) != 2 {
+		t.Fatalf("List = %d sessions, want 2", len(got))
+	}
+	if got[0].ChannelID != "direct-newest" || got[1].ChannelID != "direct-oldest" {
+		t.Errorf("List order = [%q %q], want newest first", got[0].ChannelID, got[1].ChannelID)
+	}
+}
+
+// TestPersistWritesFixedWidthTimestamps pins the write path: GetOrCreate must
+// store created_at/updated_at with the fixed-width layout so channel_sessions
+// text ordering matches time ordering. RFC3339Nano would drop trailing zeros
+// and persist a variable-width value instead.
+func TestPersistWritesFixedWidthTimestamps(t *testing.T) {
+	t.Parallel()
+
+	m, handle := newTestManager(t)
+	ctx := context.Background()
+
+	if _, err := m.GetOrCreate(ctx, directMsg("o-100"), testOwner()); err != nil {
+		t.Fatalf("GetOrCreate = %v", err)
+	}
+	var created, updated string
+	if err := handle.QueryRowContext(ctx,
+		`SELECT created_at, updated_at FROM channel_sessions WHERE channel_id = ?`,
+		"direct-o-100").Scan(&created, &updated); err != nil {
+		t.Fatalf("read stored timestamps: %v", err)
+	}
+	if len(created) != sessionTimeLen || len(updated) != sessionTimeLen {
+		t.Errorf("stored timestamps = %q/%q (%d/%d bytes), want fixed width %d",
+			created, updated, len(created), len(updated), sessionTimeLen)
+	}
+	if created != updated {
+		t.Errorf("created_at %q != updated_at %q, want the same value on insert", created, updated)
+	}
+}