feat(processing): 快照流程锁内复核收敛 + 异常显式清除语义 + 语义矩阵定稿 (ACM2-29 P2-5/M3)

- SnapshotFlow 收敛到 v2 §5 事务顺序：锁外仅解析校验；取得 PIPELINE_LOCK 后
  锁内复核快照身份（last_message_id 重放短路）→ 锁内读代与当前航班态 →
  计算 nextState → 写入 → CAS → 事件 → SUCCEEDED。锁内 CAS 失败属数据异常，
  一律回滚 FAILED(INFRA)，删除「版本号相同即视为重放」的推断路径（评审 F7/F8 收口）
- M3/F4：异常对象（FDIV/FRET/FLAB）显式清除语义——null 字面量/空对象/空串
  翻译为 Clear 命令；FlightNextState.clearedKeys 携带本次清除键；增量路径按
  映射把前缀标量列置 NULL；库内清除后读视图无该键（线格式保持键缺失，显式
  清除表达留待阶段 2 按下游契约定）
- StubFlightSchd 增量分支改为 nextState 权威整体替换（引擎已合并当前态），
  天然承载 Clear；flattenScalarsOnly 去除冗余循环（L2）
- docs/flight-state-semantics.md：16 类结构语义矩阵定稿（P0-A 交付物落库）
- 新测试：引擎 0 标记/异常清除/混排 fail-fast 单测；JDBC 异常清除回环用例

验证：MSGX_PG_PORT=5433 真实 PG ./gradlew test --rerun-tasks
105 用例 0 失败 0 跳过

src/test/kotlin/com/gzzn/omms/msgexchange/domain/flight/FlightStateEngineTest.kt

@@ -142,4 +142,40 @@ class FlightStateEngineTest {
         val next = FlightStateEngine.apply(current, commands, "m2", bumpVersion = true)
         assertEquals(emptyList<Map<String, String>>(), next.collections["GTDT"])
     }
+
+    @Test
+    fun `exception null payload translates to explicit clear and apply records cleared key`() {
+        val current = FlightNextState(
+            "F1",
+            mapOf("FLNO" to "CA100", "FRET" to """{"REID":"R1","RSN":"diverted"}"""),
+            emptyMap(),
+            3L,
+            "m1",
+        )
+        val commands = FlightStateEngine.commandsFromFields("F1", mapOf("FRET" to "null"), snapshotReplace = false)
+        assertEquals(ScalarCommand.Clear, commands.scalars["FRET"])
+
+        val next = FlightStateEngine.apply(current, commands, "m2", bumpVersion = true)
+        assertFalse(next.scalars.containsKey("FRET"))
+        assertTrue(next.clearedKeys.contains("FRET"))
+        assertEquals("CA100", next.scalars["FLNO"])
+
+        // 非空载荷仍为 Set，且清空后的下一跳 Set 恢复键（clearedKeys 不跨消息携带）
+        val restored = FlightStateEngine.apply(
+            next,
+            FlightStateEngine.commandsFromFields("F1", mapOf("FRET" to """{"REID":"R2"}"""), snapshotReplace = false),
+            "m3",
+            bumpVersion = true,
+        )
+        assertEquals("""{"REID":"R2"}""", restored.scalars["FRET"])
+        assertFalse(restored.clearedKeys.contains("FRET"))
+    }
+
+    @Test
+    fun `exception clear accepts null literal empty object and empty string`() {
+        for (payload in listOf("null", "{}", "")) {
+            val commands = FlightStateEngine.commandsFromFields("F1", mapOf("FDIV" to payload), snapshotReplace = false)
+            assertEquals(ScalarCommand.Clear, commands.scalars["FDIV"], "payload=[$payload] must be Clear")
+        }
+    }
 }