feat(joinir): Phase 201-5 Pattern 2 lowerer uses JoinValueSpace

Key changes to prevent ValueId collision between frontend and lowerer:

1. loop_step params now use ConditionEnv ValueIds (Param region: 100+)
   - i_param = env.get(loop_var_name) - ensures condition lowering works
   - carrier_param_ids from CarrierInfo.join_id

2. main() params and intermediate values use alloc_local() (Local region: 1000+)
   - No collision with frontend's param allocations

3. CarrierInfo.join_id is now properly set in frontend
   - carrier.join_id = Some(carrier_join_id) during allocation

This fixes the "use of undefined value" error where frontend allocated
ValueId(100+) but lowerer used ValueId(0+), causing remapper mismatch.

Test results:
- All 821 library tests pass
- E2E: phase200d_capture_minimal.hako outputs 30 ✓
- Pattern 4: loop_continue_pattern4.hako outputs 25 ✓
- Multi-carrier: loop_continue_multi_carrier.hako outputs 100,10 ✓

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -207,12 +207,14 @@ impl MirBuilder {
             }
         }
 
-        // Phase 201: Allocate carrier ValueIds from Param region
+        // Phase 201: Allocate carrier ValueIds from Param region and SET join_id
         // This ensures carriers are also in the safe Param region
-        for carrier in &carrier_info.carriers {
-            let _carrier_join_id = join_value_space.alloc_param();
+        // CRITICAL: We must set join_id so the lowerer can access it
+        for carrier in &mut carrier_info.carriers {
+            let carrier_join_id = join_value_space.alloc_param();
+            carrier.join_id = Some(carrier_join_id);
             eprintln!("[pattern2/phase201] Allocated carrier '{}' param ID: {:?}",
-                carrier.name, _carrier_join_id);
+                carrier.name, carrier_join_id);
         }
 
         // Phase 191: Create empty body-local environment
@@ -396,6 +398,7 @@ impl MirBuilder {
             &carrier_updates,
             analysis_body,  // Phase 191/192: Pass normalized body AST for init lowering
             Some(&mut body_local_env),  // Phase 191: Pass mutable body-local environment
+            &mut join_value_space,  // Phase 201: Unified ValueId allocation (Local region)
         ) {
             Ok((module, meta)) => (module, meta),
             Err(e) => {