gap-3 test: assert resume-write behaviour, not the NOT NULL bit

- test_column_notnull_probe_sees_gap3_outcome now branches on the
  post-ensure probe: when outcome is relaxed (PG; SQLite 3.53+)
  the resume write must land and read back NULL; on older SQLite
  the ALTER is swallowed and INSERT OR IGNORE drops the row
  (version-dependent boundary, not a migration failure).
- attempt_ledger.py comments updated: ALTER COLUMN DROP NOT NULL
  works on Postgres and SQLite 3.53.0+, swallowed before that.
- PG proof summary section 3.9/8 corrected to the same boundary.

cloud/tests/test_task_attempts_unified_schema.py

@@ -150,14 +150,31 @@ class TestFreshAndMigrations(unittest.TestCase):
         self.conn.execute(GAP3_DDL)
         flags = _column_notnull(self.conn, "task_attempts")
         self.assertTrue(flags["outcome"])
-        # SQLite 上放宽语句不合法、被 ensure 吞掉：迁移本身
-        # 不抛错、列集仍齐，但 outcome 保持 NOT NULL（需重建
-        # 表才能改，见批总结诚实边界；本批只修 PG 路径）。
+        # 迁移本身不抛错、列集仍齐；真正的不变量是 ensure
+        # 之后恢复写路与实际的 schema 状态一致，而不是
+        # outcome 的 NOT NULL 位：outcome 放宽了（PG 上
+        # ALTER 生效；SQLite 3.53.0 起也支持 ALTER COLUMN
+        # DROP NOT NULL），resume 风格写行必须能写入并读回
+        # outcome 为 NULL 的行；outcome 仍 NOT NULL 时（更
+        # 早的 SQLite：ALTER 不合法、被 try/except 吞掉），
+        # 该行会被 INSERT OR IGNORE 静默丢弃——版本相关
+        # 边界，不是迁移失败。
         ensure_task_attempts_table(self.conn)
         self.assertEqual(set(_columns(self.conn)),
                          set(UNIFIED_COLUMNS))
-        self.assertTrue(
-            _column_notnull(self.conn, "task_attempts")["outcome"])
+        relaxed = not _column_notnull(
+            self.conn, "task_attempts")["outcome"]
+        resume_record(self.conn, "t-gap", 2, "platform_fresh",
+                      "worker_shutdown", -1, "second", None)
+        self.conn.commit()
+        row = _row_by_key(self.conn, "t-gap", 2)
+        if relaxed:
+            self.assertIsNotNone(row)
+            self.assertIsNone(row["outcome"])
+            latest = latest_attempt(self.conn, "t-gap")
+            self.assertEqual(latest["attempt_no"], 2)
+        else:
+            self.assertIsNone(row)
 
     def test_old_shape_migrates_with_rows_intact(self):
         self.conn.execute(OLD_DDL)