feat(joinir): Phase 202-C Pattern 4 uses JoinValueSpace, unify dual counters

# Summary
Pattern 4 (Continue) now uses JoinValueSpace for unified ValueId allocation,
eliminating the dual counter system that was prone to ValueId collisions.

# Changes
- loop_with_continue_minimal.rs: Replace value_counter + join_value_counter with JoinValueSpace
  - Param region (100+): ConditionEnv variables
  - Local region (1000+): All intermediate values (Const, BinOp, etc.)
  - Eliminated manual counter management (value_counter = 0u32, join_value_counter++)
- pattern4_with_continue.rs: Create JoinValueSpace and pass to lowerer
- Added Phase 202-C documentation explaining the unification

# Test Results
✅ cargo test --release --lib continue: 11 passed (3 ignored)
✅ E2E apps/tests/loop_continue_pattern4.hako → 25 (single carrier)
✅ E2E apps/tests/loop_continue_multi_carrier.hako → 100, 10 (multi carrier)

# Benefits
- No ValueId collision risk (disjoint regions)
- Consistent with Pattern 2 architecture (Phase 201)
- Simplified allocation logic (no manual counter tracking)
- Better maintainability (single source of truth)

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

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

src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs

@@ -292,8 +292,19 @@ impl MirBuilder {
             }
         }
 
-        // Phase 169: Call Pattern 4 lowerer with condition AST
-        let (join_module, exit_meta) = match lower_loop_with_continue_minimal(scope, condition, self, &carrier_info, &carrier_updates) {
+        // Phase 202-C: Create JoinValueSpace for unified ValueId allocation
+        use crate::mir::join_ir::lowering::join_value_space::JoinValueSpace;
+        let mut join_value_space = JoinValueSpace::new();
+
+        // Phase 169 / Phase 202-C: Call Pattern 4 lowerer with JoinValueSpace
+        let (join_module, exit_meta) = match lower_loop_with_continue_minimal(
+            scope,
+            condition,
+            self,
+            &carrier_info,
+            &carrier_updates,
+            &mut join_value_space,
+        ) {
             Ok(result) => result,
             Err(e) => {
                 // Phase 195: Use unified trace