docs(joinir): Phase 201 implementation status and key insights

Added Section 8 to phase201-join-value-space-design.md:

- 8.1: Completed tasks summary table (all 7 tasks done)
- 8.2: Key implementation insight
  - Original plan: all params from alloc_local()
  - Actual: loop_step params use ConditionEnv's ValueIds
  - Reason: condition lowering needs matching ValueIds
- 8.3: Test results (821 tests pass, E2E verified)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/development/current/main/phase201-join-value-space-design.md

@@ -258,8 +258,64 @@ They are complementary:
 | `loop_header_phi_builder.rs` | Call reserve_phi() |
 | `merge/mod.rs` | Create JoinValueSpace, pass down |
 
-## 8. References
+## 8. Implementation Status (2025-12-09)
+
+### 8.1 Completed Tasks
+
+| Task | Status | Notes |
+|------|--------|-------|
+| 201-1: Design document | ✅ Complete | This document |
+| 201-2: JoinValueSpace box | ✅ Complete | 10 unit tests, all passing |
+| 201-3: Param region migration | ✅ Complete | ConditionEnvBuilder v2 API |
+| 201-4: PHI reservation | ✅ Skipped | Not needed - lowerer uses ConditionEnv's ValueIds directly |
+| 201-5: Local region migration | ✅ Complete | Pattern 2 lowerer updated |
+| 201-6: Testing | ✅ Complete | 821 tests pass, E2E verified |
+| 201-7: Documentation | ✅ Complete | This section |
+
+### 8.2 Key Implementation Insight
+
+The original plan assumed lowerers would allocate ALL ValueIds from JoinValueSpace.
+The actual implementation is smarter:
+
+```
+Original Plan:
+  - main params: alloc_local() → 1000+
+  - loop_step params: alloc_local() → 1000+
+  - intermediates: alloc_local() → 1000+
+
+Actual Implementation:
+  - main params: alloc_local() → 1000+ (entry point slots)
+  - loop_step params: USE ConditionEnv's ValueIds → 100+ (CRITICAL!)
+  - intermediates: alloc_local() → 1000+
+```
+
+Why? Because `lower_condition_to_joinir` uses ConditionEnv to resolve variable names.
+If `loop_step.params[0]` (i_param) doesn't match `env.get("i")`, condition lowering fails.
+
+### 8.3 Test Results
+
+```
+# Library tests
+$ cargo test --release --lib
+test result: ok. 821 passed; 0 failed
+
+# E2E tests
+$ ./target/release/hakorune apps/tests/phase200d_capture_minimal.hako
+30  # ✓ Expected output
+
+$ ./target/release/hakorune apps/tests/loop_continue_pattern4.hako
+25  # ✓ Expected output
+
+$ ./target/release/hakorune apps/tests/loop_continue_multi_carrier.hako
+100
+10  # ✓ Expected output (two carriers)
+```
+
+## 9. References
 
 - Phase 201-A analysis: carrier PHI dst overwrite bug
 - joinir-architecture-overview.md: JoinIR invariants
 - value_id_ranges.rs: Module-level ValueId isolation
+- Commits:
+  - `1af53f82` feat(joinir): Phase 201 JoinValueSpace - unified ValueId allocation
+  - `17152baf` feat(joinir): Phase 201-5 Pattern 2 lowerer uses JoinValueSpace