feat(joinir): Phase 33-14 JoinFragmentMeta for expr/carrier separation

Introduces JoinFragmentMeta to distinguish between loop expression results
and carrier variable updates, fixing SSA correctness issues.

## Changes

### New: JoinFragmentMeta struct (carrier_info.rs)
- `expr_result: Option<ValueId>` - Loop as expression (return loop(...))
- `exit_meta: ExitMeta` - Carrier updates for variable_map
- Helper methods: with_expr_result(), carrier_only(), empty()

### Pattern 2 Lowerer Updates
- loop_with_break_minimal.rs: Returns (JoinModule, JoinFragmentMeta)
- pattern2_with_break.rs: Sets boundary.expr_result from fragment_meta

### instruction_rewriter.rs
- Phase 33-14: Only add to exit_phi_inputs when boundary.expr_result is Some
- Phase 33-13: MergeResult struct with carrier_inputs map

### JoinInlineBoundary (inline_boundary.rs)
- New field: expr_result: Option<ValueId>
- All constructors updated with expr_result: None default

## Design Philosophy

Previously, exit_phi_inputs mixed expr results with carrier updates, causing:
- PHI inputs referencing undefined remapped values
- SSA-undef errors in VM execution

With JoinFragmentMeta:
- expr_result → exit_phi_inputs (generates PHI for expr value)
- exit_meta → carrier_inputs (updates variable_map via carrier PHIs)

## Test Results
- Pattern 1 (carrier-only): Works correctly (no exit_phi_inputs)
- Pattern 2 (expr result): Design complete, SSA-undef fix deferred to Phase 33-15

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

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

src/mir/builder/control_flow/joinir/routing.rs

@@ -75,6 +75,7 @@ impl MirBuilder {
             // Phase 170-A-1: Test methods (simplified versions)
             "TrimTest.trim/1" => true,
             "Main.trim/1" => true,  // Phase 171-fix: Main box variant
+            "Main.trim_string_simple/1" => true,  // Phase 33-13: Simple trim variant
             _ => false,
         };