feat(control_tree): Phase 131 P1.5-P2 DirectValue exit reconnection

Implement DirectValue mode for Normalized shadow exit handling:

**P1.5 Changes**:
- Add ExitReconnectMode::DirectValue (skip exit PHI generation)
- Carry remapped_exit_values through merge result
- Update host variable_map directly with exit values
- Fix loop(true) { x = 1; break }; return x to return 1 correctly

**P2 Changes**:
- Normalize k_exit continuation entry/exit edges
- Rewrite TailCall(k_exit) → Jump(exit_block) for proper merge
- Add verify_all_terminator_targets_exist contract check
- Extend ExitLineReconnector to handle DirectValue mode

**Infrastructure**:
- tools/build_llvm.sh: Force TMPDIR under target/ (EXDEV mitigation)
- llvm_exe_runner.sh: Add exit_code verification support
- Phase 131 smokes: Update for dev-only + exit code validation

**Contracts**:
- PHI-free: Normalized path uses continuations only
- Exit values reconnect via remapped ValueIds
- Existing patterns unaffected (既定挙動不変)

Related: Phase 131 loop(true) break-once Normalized support

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

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

src/mir/control_tree/normalized_shadow/mod.rs

@@ -40,8 +40,10 @@ pub mod legacy;
 pub mod dev_pipeline;
 pub mod parity_contract;
 pub mod available_inputs_collector; // Phase 126: available_inputs SSOT
+pub mod exit_reconnector; // Phase 131 P1.5: Direct variable_map reconnection (Option B)
 
 pub use builder::StepTreeNormalizedShadowLowererBox;
 pub use contracts::{CapabilityCheckResult, UnsupportedCapability};
 pub use parity_contract::{MismatchKind, ShadowParityResult};
 pub use env_layout::EnvLayout;
+pub use exit_reconnector::ExitReconnectorBox; // Phase 131 P1.5