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/timefmt.go

@@ -0,0 +1,21 @@
+package session
+
+import "time"
+
+// sessionTimeLayout is the fixed-width timestamp layout used for every stored
+// created_at/updated_at value. Unlike time.RFC3339Nano it never drops trailing
+// fractional zeros, so every value is exactly sessionTimeLen bytes wide. SQLite
+// compares these TEXT columns as plain strings, and equal width is what makes
+// that comparison agree with chronological order. Values are written in UTC, so
+// the Z07:00 zone renders as a single "Z".
+const sessionTimeLayout = "2006-01-02T15:04:05.000000000Z07:00"
+
+// sessionTimeLen is the byte length of a UTC value formatted with
+// sessionTimeLayout: "2006-01-02T15:04:05.000000000Z" — the Z07:00 zone
+// collapses to "Z" in UTC.
+const sessionTimeLen = 30
+
+// formatSessionTime encodes t in UTC with the fixed-width sessionTimeLayout.
+func formatSessionTime(t time.Time) string {
+	return t.UTC().Format(sessionTimeLayout)
+}