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/backend/mir_interpreter/mod.rs

@@ -6,7 +6,7 @@
  * Print/Debug (best-effort), Barrier/Safepoint (no-op).
  */
 
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 use crate::box_trait::NyashBox;
 
@@ -31,7 +31,7 @@ pub struct MirInterpreter {
     pub(super) mem: HashMap<ValueId, VMValue>,
     // Object field storage keyed by stable object identity (Arc ptr addr fallback)
     pub(super) obj_fields: HashMap<u64, HashMap<String, VMValue>>,
-    pub(super) functions: HashMap<String, MirFunction>,
+    pub(super) functions: BTreeMap<String, MirFunction>,
     pub(super) cur_fn: Option<String>,
     // Trace context (dev-only; enabled with NYASH_VM_TRACE=1)
     pub(super) last_block: Option<BasicBlockId>,
@@ -54,7 +54,7 @@ impl MirInterpreter {
             regs: HashMap::new(),
             mem: HashMap::new(),
             obj_fields: HashMap::new(),
-            functions: HashMap::new(),
+            functions: BTreeMap::new(),
             cur_fn: None,
             last_block: None,
             last_inst: None,