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.go

@@ -19,6 +19,11 @@ import (
 // channel maps to exactly one internal session_id. created_at/updated_at follow
 // the project's timestamp convention; actor_role records the identity lane so the
 // mapping survives restarts.
+//
+// The created_at/updated_at DEFAULT expressions are a fallback only: every write
+// path (insertOrIgnoreSessionSQL) supplies explicit fixed-width UTC timestamps
+// via formatSessionTime, so the 3-digit-millisecond strftime defaults are never
+// exercised and no row mixes timestamp widths.
 const createTableSQL = `
 CREATE TABLE IF NOT EXISTS channel_sessions (
     platform    TEXT NOT NULL,
@@ -43,10 +48,13 @@ SELECT session_id FROM channel_sessions WHERE platform = ? AND channel_id = ?`
 
 // listSessionsSQL selects every persisted channel mapping, newest first. Group
 // sessions key on a channel_id carrying groupChannelPrefix; direct sessions use
-// their channel id.
+// their channel id. created_at is fixed-width UTC text (see sessionTimeLayout),
+// so its text order is its time order; rowid breaks created_at ties so mappings
+// written in the same coarse clock tick still list in a stable order
+// (channel_sessions is an ordinary rowid table, not WITHOUT ROWID).
 const listSessionSQL = `
 SELECT platform, channel_id, session_id, actor_role FROM channel_sessions
-ORDER BY created_at DESC`
+ORDER BY created_at DESC, rowid DESC`
 
 // groupChannelPrefix marks a channel id as a group conversation so a session
 // reconstructed from the backing table (where no IsGroup flag is persisted) can
@@ -178,7 +186,7 @@ func (m *SessionManager) lookupCached(key string) (Session, bool) {
 // winning writer's session_id becomes the binding. It is called under the write
 // lock so the in-memory cache and the table stay in agreement.
 func (m *SessionManager) persist(ctx context.Context, platform, cid string, actor adapter.Actor) (string, error) {
-	now := time.Now().UTC().Format(time.RFC3339Nano)
+	now := formatSessionTime(time.Now())
 	if _, err := m.db.ExecContext(ctx, insertOrIgnoreSessionSQL, platform, cid, newSessionID(), string(actor.Role), now, now); err != nil {
 		return "", fmt.Errorf("session: persist %s/%s: %w", platform, cid, err)
 	}