fix(joinir): Use BTreeMap for MirModule.functions deterministic iteration

HashMap iteration order is non-deterministic due to HashDoS protection.
This caused merge_joinir_mir_blocks to sometimes process functions in
wrong order, leading to incorrect block remapping.

Changed:
- MirModule.functions: HashMap → BTreeMap
- MirInterpreter.functions: HashMap → BTreeMap
- IfLoweringDryRunner.scan_module: HashMap → BTreeMap parameter

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

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

src/mir/join_ir/lowering/if_dry_runner.rs

@@ -11,7 +11,7 @@
 
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{MirFunction, MirInstruction};
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::time::{Duration, Instant};
 
 /// If lowering dry-run スキャナー
@@ -42,7 +42,7 @@ impl IfLoweringDryRunner {
     /// - 各 Branch ブロックで try_lower_if_to_joinir() 試行
     /// - パフォーマンス計測（マイクロ秒レベル）
     /// - 統計情報収集（Select/IfMerge分類）
-    pub fn scan_module(&self, functions: &HashMap<String, MirFunction>) -> DryRunStats {
+    pub fn scan_module(&self, functions: &BTreeMap<String, MirFunction>) -> DryRunStats {
         let mut total_branches = 0;
         let mut lowered_count = 0;
         let mut select_count = 0;