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/runtime/persist/persist.go

@@ -94,9 +94,13 @@ const (
 	loadSQL = `SELECT id, agent_id, step, state, created_at, updated_at
 		FROM checkpoints WHERE id = ?`
 
+	// updated_at is stored as fixed-width UTC text (see formatTime), so its text
+	// order is its time order. step breaks ties between checkpoints written in
+	// the same tick, and rowid makes that tiebreak deterministic (checkpoints is
+	// an ordinary rowid table, not WITHOUT ROWID).
 	loadLatestSQL = `SELECT id, agent_id, step, state, created_at, updated_at
 		FROM checkpoints WHERE agent_id = ?
-		ORDER BY updated_at DESC, step DESC LIMIT 1`
+		ORDER BY updated_at DESC, step DESC, rowid DESC LIMIT 1`
 
 	listSQL = `SELECT id, agent_id, step, state, created_at, updated_at
 		FROM checkpoints WHERE agent_id = ?
@@ -112,6 +116,8 @@ const (
 // which already constrains the pool to a single connection for write
 // serialization. The caller owns db and must Close it when done. log may be nil
 // and is replaced with a discard logger so the Store can always log safely.
+// Legacy variable-width timestamps are normalized in place (see
+// normalizeTimestamps) so text ordering matches time ordering.
 func New(ctx context.Context, db *sql.DB, log *slog.Logger) (*Store, error) {
 	if db == nil {
 		return nil, fmt.Errorf("persist: new: %w", ErrNilDB)
@@ -124,6 +130,9 @@ func New(ctx context.Context, db *sql.DB, log *slog.Logger) (*Store, error) {
 	if _, err := db.ExecContext(ctx, createIndexSQL); err != nil {
 		return nil, fmt.Errorf("persist: create index: %w", err)
 	}
+	if err := normalizeTimestamps(ctx, db); err != nil {
+		return nil, err
+	}
 	return &Store{db: db, log: log}, nil
 }
 
@@ -270,16 +279,6 @@ func scanCheckpoint(sc rowScanner) (*Checkpoint, error) {
 	}, nil
 }
 
-// formatTime renders t as a UTC RFC3339 timestamp with nanosecond precision.
-func formatTime(t time.Time) string {
-	return t.UTC().Format(time.RFC3339Nano)
-}
-
-// parseTime parses a timestamp written by formatTime.
-func parseTime(s string) (time.Time, error) {
-	return time.Parse(time.RFC3339Nano, s)
-}
-
 // discardLogger returns log if non-nil, otherwise a logger that writes nowhere,
 // so the Store can always log safely.
 func discardLogger(log *slog.Logger) *slog.Logger {