feat(joinir): Phase 197 Pattern4 multi-carrier exit fix & AST-based update

Phase 197-B: Multi-Carrier Exit Mechanism
- Fixed reconnect_boundary() to use remapper for per-carrier exit values
- ExitMeta now uses carrier_param_ids (Jump arguments) instead of
  carrier_exit_ids (k_exit parameters)
- Root cause: k_exit parameters aren't defined when JoinIR functions
  merge into host MIR

Phase 197-C: AST-Based Update Expression
- LoopUpdateAnalyzer extracts update patterns from loop body AST
- Pattern4 lowerer uses UpdateExpr for semantically correct RHS
- sum = sum + i → uses i_next (current iteration value)
- count = count + 1 → uses const_1

Files modified:
- src/mir/builder/control_flow/joinir/merge/mod.rs
- src/mir/join_ir/lowering/loop_with_continue_minimal.rs
- src/mir/join_ir/lowering/loop_update_analyzer.rs (new)

Test results:
- loop_continue_multi_carrier.hako: 25, 5 ✅
- loop_continue_pattern4.hako: 25 ✅

Pattern 1–4 now unified with:
- Structure-based detection (LoopFeatures + classify)
- Carrier/Exit metadata (CarrierInfo + ExitMeta)
- Boundary connection (JoinInlineBoundary + LoopExitBinding)
- AST-based update expressions (LoopUpdateAnalyzer)

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

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

src/mir/join_ir/lowering/mod.rs

@@ -17,6 +17,7 @@
 
 pub mod carrier_info; // Phase 196: Carrier metadata for loop lowering
 pub mod common;
+pub mod loop_update_analyzer; // Phase 197: Update expression analyzer for carrier semantics
 pub mod exit_args_resolver;
 pub mod funcscanner_append_defs;
 pub mod funcscanner_trim;