refactor(joinir): extract ExitArgsCollectorBox for jump_args SSOT

Phase 118 refactoring: Box modularization for exit value collection.

# Changes
- Created `exit_args_collector.rs` with ExitArgsCollectorBox
- Extracted jump_args processing logic from instruction_rewriter.rs
- Implemented SSOT for offset calculation (0 vs 1 legacy layout)
- Added comprehensive unit tests (6 tests, all passing)

# Design
- **Responsibility**: Collects exit PHI inputs + carrier inputs from jump_args
- **SSOT**: Centralized offset calculation logic
- **Fail-Fast**: Validates jump_args length against exit_bindings
- **Phase 118 P2 Contract**: Uses boundary.exit_bindings as SSOT

# Key Features
- Filters ConditionOnly carriers (no exit PHI needed)
- Handles both direct mapping (offset=0) and legacy layout (offset=1)
- Strict mode: Fail-Fast on validation errors
- Non-strict mode: Warns and continues with best effort

# Test Results
- Unit tests: 6/6 PASS
- Phase 118 smoke tests: PASS (VM + LLVM)
- Phase 117 regression: PASS

# Reduced Code Complexity
- instruction_rewriter.rs: ~90 lines → ~25 lines (box call)
- Offset logic: Centralized in ExitArgsCollectorBox

🤖 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/merge/mod.rs

@@ -15,6 +15,7 @@
 mod block_allocator;
 mod carrier_init_builder;
 mod contract_checks;
+pub mod exit_args_collector; // Phase 118: Exit args collection box
 pub mod exit_line;
 mod exit_phi_builder;
 mod expr_result_resolver;