feat(mir): Phase 69-3 Fix MIR non-determinism with BTreeSet

Replace HashSet with BTreeSet for CFG predecessors/successors:
- BasicBlock.predecessors: HashSet → BTreeSet
- BasicBlock.successors: HashSet → BTreeSet
- LoopFormOps.get_block_predecessors(): returns BTreeSet
- BasicBlock.dominates(): takes &[BTreeSet<BasicBlockId>]

This ensures deterministic PHI generation and test stability.

Test results:
- loop_with_continue_and_break tests: now deterministic (3/3 same output)
- loopform tests: 14/14 PASS (no regressions)
- merge_exit_with_classification tests: 3/3 PASS

Technical changes (6 files):
- basic_block.rs: BTreeSet types + new() initialization
- loopform_builder.rs: trait signature + 2 mock implementations
- phi_ops.rs: return type
- json_v0_bridge/loop_.rs: return type

Same pattern as Phase 25.1 (MirFunction.blocks HashMap → BTreeMap).

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

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

src/mir/loop_builder/if_in_loop_phi_emitter.rs

@@ -68,12 +68,13 @@ impl IfInLoopPhiEmitter {
         if_shape: &IfShape,
     ) -> Result<usize, String> {
         let mut phi_count = 0;
-        let trace_on = std::env::var("HAKO_JOINIR_IF_IN_LOOP_TRACE").ok().as_deref() == Some("1");
+        let trace_on = std::env::var("HAKO_JOINIR_IF_IN_LOOP_TRACE")
+            .ok()
+            .as_deref()
+            == Some("1");
 
         if trace_on {
-            eprintln!(
-                "[Phase 61-3] IfInLoopPhiEmitter::emit_header_phis start"
-            );
+            eprintln!("[Phase 61-3] IfInLoopPhiEmitter::emit_header_phis start");
             eprintln!("[Phase 61-3]   header_phis: {:?}", phi_spec.header_phis);
             eprintln!("[Phase 61-3]   carrier_names: {:?}", carrier_names);
         }
@@ -99,10 +100,7 @@ impl IfInLoopPhiEmitter {
             };
 
             // Then値: snapshot から取得、なければ pre_val
-            let then_val = then_snapshot
-                .get(var_name)
-                .copied()
-                .unwrap_or(pre_val);
+            let then_val = then_snapshot.get(var_name).copied().unwrap_or(pre_val);
 
             // Else値: snapshot から取得、なければ pre_val（片腕 PHI パターン）
             let else_val = else_snapshot_opt
@@ -181,8 +179,10 @@ impl IfInLoopPhiEmitter {
         if_shape: &IfShape,
     ) -> Result<usize, String> {
         let mut phi_count = 0;
-        let trace_on =
-            std::env::var("HAKO_JOINIR_IF_TOPLEVEL_TRACE").ok().as_deref() == Some("1");
+        let trace_on = std::env::var("HAKO_JOINIR_IF_TOPLEVEL_TRACE")
+            .ok()
+            .as_deref()
+            == Some("1");
 
         if trace_on {
             eprintln!("[Phase 61-4] IfInLoopPhiEmitter::emit_toplevel_phis start");

src/mir/loop_builder/if_lowering.rs

@@ -203,7 +203,8 @@ impl<'a> LoopBuilder<'a> {
         // Phase 62-B: JoinIRIfPhiSelector箱化（-60行の簡潔化）
         let joinir_result = if crate::config::env::joinir_if_select_enabled() {
             if let Some(ref func) = self.parent_builder.current_function {
-                let selector = super::JoinIRIfPhiSelector::new(func, pre_branch_bb, carrier_names.clone());
+                let selector =
+                    super::JoinIRIfPhiSelector::new(func, pre_branch_bb, carrier_names.clone());
                 selector.try_lower()
             } else {
                 super::JoinIRResult {

src/mir/loop_builder/loop_form.rs

@@ -395,7 +395,7 @@ impl<'a> LoopBuilder<'a> {
                             merge_block.add_instruction(MirInstruction::Phi {
                                 dst: phi_id,
                                 inputs: final_inputs,
-                type_hint: None, // Phase 63-6
+                                type_hint: None, // Phase 63-6
                             });
                         }
                     }

src/mir/loop_builder/phi_ops.rs

@@ -172,16 +172,17 @@ impl<'a> LoopFormOps for LoopBuilder<'a> {
     fn get_block_predecessors(
         &self,
         block: BasicBlockId,
-    ) -> std::collections::HashSet<BasicBlockId> {
+    ) -> std::collections::BTreeSet<BasicBlockId> {
         // 📦 Hotfix 6: Get actual CFG predecessors for PHI validation
+        // Phase 69-3: Changed to BTreeSet for determinism
         if let Some(ref func) = self.parent_builder.current_function {
             if let Some(bb) = func.blocks.get(&block) {
                 bb.predecessors.clone()
             } else {
-                std::collections::HashSet::new() // Non-existent blocks have no predecessors
+                std::collections::BTreeSet::new() // Non-existent blocks have no predecessors
             }
         } else {
-            std::collections::HashSet::new()
+            std::collections::BTreeSet::new()
         }
     }