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/function.rs

@@ -5,7 +5,7 @@
  */
 
 use super::{BasicBlock, BasicBlockId, EffectMask, MirInstruction, MirType, ValueId};
-use std::collections::{HashMap, HashSet};
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::fmt;
 
 /// Function signature for MIR functions
@@ -381,8 +381,8 @@ pub struct MirModule {
     /// Module name
     pub name: String,
 
-    /// Functions in this module
-    pub functions: HashMap<String, MirFunction>,
+    /// Functions in this module (BTreeMap for deterministic iteration order)
+    pub functions: BTreeMap<String, MirFunction>,
 
     /// Global constants/statics
     pub globals: HashMap<String, super::ConstValue>,
@@ -416,7 +416,7 @@ impl MirModule {
     pub fn new(name: String) -> Self {
         Self {
             name,
-            functions: HashMap::new(),
+            functions: BTreeMap::new(),
             globals: HashMap::new(),
             metadata: ModuleMetadata::default(),
         }