refactor(joinir): Phase 222.5-E - Merge/Boundary HashMap → BTreeMap

Convert 10 medium-priority locations to BTreeMap for determinism:

Merge System (6 locations):
- instruction_rewriter.rs: value_to_func_name, function_params,
  function_entry_map, local_block_map
- value_collector.rs: value_to_func_name, function_params,
  function_entry_map

ID Remapper (2 locations):
- joinir_id_remapper.rs: block_map, value_map

Boundary Injector (2 locations):
- joinir_inline_boundary_injector.rs: value_map (param & return)

Impact:
- Changed files: 4
- Changed lines: +36, -24 (net +12)
- Tests: 849/856 PASS (no regression)
- Determinism: Merge/Boundary processing now deterministic

Combined with Phase 222.5-D (13 high-priority locations),
JoinIR pipeline now uses BTreeMap uniformly across all
critical paths (Pattern/Merge/Boundary/ValueId allocation).

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

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

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -13,7 +13,7 @@ use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
 use super::loop_header_phi_info::LoopHeaderPhiInfo;
 use super::tail_call_classifier::{TailCallKind, classify_tail_call};
 use super::merge_result::MergeResult;
-use std::collections::HashMap;
+use std::collections::BTreeMap;  // Phase 222.5-E: HashMap → BTreeMap for determinism
 
 /// Phase 179-A: Exit continuation function name (MIR convention)
 /// This is the standard name for k_exit continuations in JoinIR → MIR lowering
@@ -44,8 +44,8 @@ pub(super) fn merge_and_rewrite(
     builder: &mut crate::mir::builder::MirBuilder,
     mir_module: &MirModule,
     remapper: &mut JoinIrIdRemapper,
-    value_to_func_name: &HashMap<ValueId, String>,
-    function_params: &HashMap<String, Vec<ValueId>>,
+    value_to_func_name: &BTreeMap<ValueId, String>,  // Phase 222.5-E: HashMap → BTreeMap for determinism
+    function_params: &BTreeMap<String, Vec<ValueId>>,  // Phase 222.5-E: HashMap → BTreeMap for determinism
     boundary: Option<&JoinInlineBoundary>,
     loop_header_phi_info: &mut LoopHeaderPhiInfo,
     exit_block_id: BasicBlockId,
@@ -68,7 +68,8 @@ pub(super) fn merge_and_rewrite(
         std::collections::BTreeMap::new();
 
     // Build function_entry_map for Call→Jump conversion
-    let mut function_entry_map: HashMap<String, BasicBlockId> = HashMap::new();
+    // Phase 222.5-E: HashMap → BTreeMap for determinism
+    let mut function_entry_map: BTreeMap<String, BasicBlockId> = BTreeMap::new();
     for (func_name, func) in &mir_module.functions {
         let entry_block_new = remapper
             .get_block(func_name, func.entry_block)
@@ -121,7 +122,8 @@ pub(super) fn merge_and_rewrite(
         }
 
         // Build a local block map for this function (for remap_instruction compatibility)
-        let mut local_block_map: HashMap<BasicBlockId, BasicBlockId> = HashMap::new();
+        // Phase 222.5-E: HashMap → BTreeMap for determinism
+        let mut local_block_map: BTreeMap<BasicBlockId, BasicBlockId> = BTreeMap::new();
         for old_block_id in func.blocks.keys() {
             let new_block_id = remapper
                 .get_block(func_name, *old_block_id)
@@ -687,8 +689,9 @@ pub(super) fn merge_and_rewrite(
             .get_block(entry_func_name, entry_func.entry_block)
             .ok_or_else(|| format!("Entry block not found for {}", entry_func_name))?;
 
-        // Create HashMap from remapper for BoundaryInjector (temporary adapter)
-        let mut value_map_for_injector = HashMap::new();
+        // Create BTreeMap from remapper for BoundaryInjector (temporary adapter)
+        // Phase 222.5-E: HashMap → BTreeMap for determinism
+        let mut value_map_for_injector = BTreeMap::new();
 
         // Phase 171-fix: Add join_inputs to value_map
         for join_in in &boundary.join_inputs {

src/mir/builder/control_flow/joinir/merge/value_collector.rs

@@ -7,7 +7,8 @@
 
 use crate::mir::{MirInstruction, MirModule, ValueId};
 use crate::mir::builder::joinir_id_remapper::JoinIrIdRemapper;
-use std::collections::{BTreeSet, HashMap};
+use std::collections::BTreeSet;  // Phase 222.5-E: HashMap → BTreeMap for determinism
+use std::collections::BTreeMap;  // Phase 222.5-E: HashMap → BTreeMap for determinism
 
 /// Phase 2: Collect all ValueIds used across ALL functions (Phase 189)
 ///
@@ -21,8 +22,8 @@ pub(super) fn collect_values(
 ) -> Result<
     (
         BTreeSet<ValueId>,
-        HashMap<ValueId, String>,
-        HashMap<String, Vec<ValueId>>,
+        BTreeMap<ValueId, String>,  // Phase 222.5-E: HashMap → BTreeMap for determinism
+        BTreeMap<String, Vec<ValueId>>,  // Phase 222.5-E: HashMap → BTreeMap for determinism
     ),
     String,
 > {
@@ -33,11 +34,13 @@ pub(super) fn collect_values(
     }
 
     let mut used_values: BTreeSet<ValueId> = BTreeSet::new();
-    let mut value_to_func_name: HashMap<ValueId, String> = HashMap::new();
-    let mut function_params: HashMap<String, Vec<ValueId>> = HashMap::new();
+    // Phase 222.5-E: HashMap → BTreeMap for determinism
+    let mut value_to_func_name: BTreeMap<ValueId, String> = BTreeMap::new();
+    let mut function_params: BTreeMap<String, Vec<ValueId>> = BTreeMap::new();
 
     // Build function_entry_map for tracking function names
-    let function_entry_map: HashMap<String, ()> = mir_module
+    // Phase 222.5-E: HashMap → BTreeMap for determinism
+    let function_entry_map: BTreeMap<String, ()> = mir_module
         .functions
         .keys()
         .map(|name| (name.clone(), ()))