feat(joinir): Phase 176 Pattern2 multi-carrier lowering complete

Task 176-1: Pattern2 limitation investigation
- Identified 10 limitation points where only position carrier was handled
- Added TODO markers for Phase 176-2/3 implementation
- Created phase176-pattern2-limitations.md documentation

Task 176-2: CarrierUpdateLowerer helper implementation
- Implemented emit_carrier_update() helper function
- Supports CounterLike and AccumulationLike UpdateExpr patterns
- Added 6 unit tests (all passing)
- Fail-Fast error handling for carrier/variable not found

Task 176-3: Pattern2 lowerer multi-carrier extension
- Extended header PHI generation for all carriers
- Implemented loop update for all carriers using emit_carrier_update()
- Extended ExitLine/ExitMeta construction for all carriers
- Updated function call/jump args to include all carriers
- 9/10 tests passing (1 pre-existing test issue)

Task 176-4: E2E testing and bug fixes
- Fixed Trim pattern loop_var_name overwrite bug (pattern2_with_break.rs)
- Fixed InstructionRewriter latch_incoming mapping bug
- All E2E tests passing (RC=0): pos + result dual-carrier loops work
- test_jsonparser_parse_string_min2.hako verified

Task 176-5: Documentation updates
- Created phase176-completion-report.md
- Updated phase175-multicarrier-design.md with completion status
- Updated joinir-architecture-overview.md roadmap
- Updated CURRENT_TASK.md with Phase 176 completion + Phase 177 TODO
- Updated loop_pattern_space.md F-axis (multi-carrier support complete)

Technical achievements:
- Pattern2 now handles single/multiple carriers uniformly
- CarrierInfo architecture proven to work end-to-end
- Two critical bugs fixed (loop_var overwrite, latch_incoming mapping)
- No regressions in existing tests

Next: Phase 177 - Apply to JsonParser _parse_string full implementation

src/mir/builder/control_flow/joinir/merge/mod.rs

@@ -157,6 +157,18 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
 
             // Phase 33-20: Extract other carriers from exit_bindings
             // Skip the loop variable (it's handled separately) and collect other carriers
+            eprintln!(
+                "[cf_loop/joinir] Phase 33-20 DEBUG: exit_bindings count={}, loop_var_name={:?}",
+                boundary.exit_bindings.len(),
+                loop_var_name
+            );
+            for b in boundary.exit_bindings.iter() {
+                eprintln!(
+                    "[cf_loop/joinir] Phase 33-20 DEBUG:   exit_binding: carrier_name={:?}, host_slot={:?}",
+                    b.carrier_name, b.host_slot
+                );
+            }
+
             let other_carriers: Vec<(String, ValueId)> = boundary.exit_bindings
                 .iter()
                 .filter(|b| b.carrier_name != *loop_var_name)