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/builder/control_flow.rs

@@ -542,7 +542,11 @@ impl super::MirBuilder {
                     values.insert(*v);
                 }
             }
-            MirInstruction::Phi { dst, inputs, type_hint: None } => {
+            MirInstruction::Phi {
+                dst,
+                inputs,
+                type_hint: None,
+            } => {
                 values.insert(*dst);
                 for (_, val) in inputs {
                     values.insert(*val);
@@ -651,7 +655,11 @@ impl super::MirBuilder {
             MirInstruction::Return { value } => MirInstruction::Return {
                 value: value.map(remap_value),
             },
-            MirInstruction::Phi { dst, inputs, type_hint: None } => MirInstruction::Phi {
+            MirInstruction::Phi {
+                dst,
+                inputs,
+                type_hint: None,
+            } => MirInstruction::Phi {
                 dst: remap_value(*dst),
                 inputs: inputs
                     .iter()

src/mir/builder/if_form.rs

@@ -22,7 +22,8 @@ impl<'a> PhiBuilderOps for ToplevelOps<'a> {
         inputs: Vec<(BasicBlockId, ValueId)>,
     ) -> Result<(), String> {
         // merge ブロックの先頭に PHI を挿入
-        if let (Some(func), Some(_cur_bb)) = (self.0.current_function.as_mut(), self.0.current_block)
+        if let (Some(func), Some(_cur_bb)) =
+            (self.0.current_function.as_mut(), self.0.current_block)
         {
             crate::mir::ssot::cf_common::insert_phi_at_head_spanned(
                 func,
@@ -32,7 +33,11 @@ impl<'a> PhiBuilderOps for ToplevelOps<'a> {
                 self.0.current_span,
             );
         } else {
-            self.0.emit_instruction(MirInstruction::Phi { dst, inputs, type_hint: None })?;
+            self.0.emit_instruction(MirInstruction::Phi {
+                dst,
+                inputs,
+                type_hint: None,
+            })?;
         }
         Ok(())
     }
@@ -286,7 +291,10 @@ impl MirBuilder {
                     }
                     None => {
                         if joinir_dryrun {
-                            eprintln!("[Phase 61-4] ⏭️ JoinIR pattern not matched for {}, using fallback", func_name);
+                            eprintln!(
+                                "[Phase 61-4] ⏭️ JoinIR pattern not matched for {}, using fallback",
+                                func_name
+                            );
                         }
                     }
                 }