docs: mark Step 2.4 as complete in PLAN.md

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PLAN.md

@@ -81,11 +81,11 @@ Check off items as completed. Each phase builds on the previous.
 - [x] **Commit checkpoint** (80784ff)
 
 ### Step 2.4: Projection Infrastructure
-- [ ] Create `projections/base.py` with Projection base class
-- [ ] Create ProjectionRegistry
-- [ ] Create `events/processor.py` with process_event function
-- [ ] Write tests for projection registration and processing
-- [ ] **Commit checkpoint**
+- [x] Create `projections/base.py` with Projection base class
+- [x] Create ProjectionRegistry
+- [x] Create `events/processor.py` with process_event function
+- [x] Write tests for projection registration and processing
+- [x] **Commit checkpoint** (42cb1ed)
 
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