feat(01-01): implement WorkflowStateManager with atomic write

GREEN phase: Implement state management with:
- Atomic write pattern (temp file + os.replace)
- Backup rotation (workflow_state.json -> .bak)
- Backup fallback on corruption
- get_or_create idempotent creation

Fix test to properly create backup before corruption test.

All 20 tests pass.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_pipeline/test_state.py

@@ -281,7 +281,7 @@ class TestBackupFallback:
         state_file = tmp_path / "workflow_state.json"
         backup_file = tmp_path / "workflow_state.json.bak"
 
-        # Create valid backup
+        # Create valid backup by saving twice (backup is created on second save)
         valid_state = WorkflowState(
             content_id="backup-valid-001",
             platform="bilibili",
@@ -289,11 +289,22 @@ class TestBackupFallback:
         )
         manager.save(valid_state)
 
+        # Save again to create backup
+        valid_state2 = WorkflowState(
+            content_id="backup-valid-002",
+            platform="bilibili",
+            source_url="https://www.bilibili.com/video/BV888"
+        )
+        manager.save(valid_state2)
+
+        # Now backup exists with "backup-valid-001", main has "backup-valid-002"
+        assert backup_file.exists()
+
         # Corrupt main file
         with open(state_file, 'w', encoding='utf-8') as f:
             f.write("{ invalid json content }")
 
-        # Should fall back to backup
+        # Should fall back to backup (which has backup-valid-001)
         loaded = manager.load()
 
         assert loaded is not None