refactor(ingress): 移除 G1 窗口补偿扫描与迟到到达检测

实际负载不足 10 条/秒，G1 属过度防御。删除迟到到达检测机制、
existingIds 接口、PipelineCounters 字段、lateDetect 配置，
以及文档中 CLM-1/CLM-2 声明与 G1 缺口索引。

src/test/kotlin/com/gzzn/omms/msgexchange/processing/BackfillServiceTest.kt

@@ -49,7 +49,6 @@ class BackfillServiceTest {
         override fun readRange(fromExclusive: Long, limit: Int): List<MailboxRow> = emptyList()
         override fun maxId(): Long? = null
         override fun minId(): Long? = null
-        override fun existingIds(msgIds: Collection<Long>): Set<Long> = emptySet()
         override fun markProcessedIfUnmarked(msgId: Long, value: String): MailboxMarkResult {
             if (fail) throw IllegalStateException("mysql-down")
             if (missing) return MailboxMarkResult.MISSING
@@ -240,7 +239,6 @@ class BackfillServiceTest {
             override fun readRange(fromExclusive: Long, limit: Int) = emptyList<MailboxRow>()
             override fun maxId(): Long? = 1L
             override fun minId(): Long? = 1L
-            override fun existingIds(msgIds: Collection<Long>): Set<Long> = emptySet()
             override fun markProcessedIfUnmarked(msgId: Long, value: String): MailboxMarkResult {
                 entered.countDown()
                 release.await()