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/phi_core/loopform_builder.rs

@@ -869,10 +869,11 @@ pub trait LoopFormOps {
     /// 📦 Get actual CFG predecessors for a block (Hotfix 6: PHI input validation)
     /// Returns the set of blocks that actually branch to this block in the CFG.
     /// Used to validate exit PHI inputs against actual control flow.
+    /// Phase 69-3: Changed to BTreeSet for determinism
     fn get_block_predecessors(
         &self,
         block: BasicBlockId,
-    ) -> std::collections::HashSet<BasicBlockId>;
+    ) -> std::collections::BTreeSet<BasicBlockId>;
 
     /// Phase 26-A-4: ValueIdベースのパラメータ判定（型安全化）
     ///
@@ -1029,9 +1030,10 @@ mod tests {
             fn get_block_predecessors(
                 &self,
                 _block: BasicBlockId,
-            ) -> std::collections::HashSet<BasicBlockId> {
+            ) -> std::collections::BTreeSet<BasicBlockId> {
                 // MockOps: return empty set (no CFG in test)
-                std::collections::HashSet::new()
+                // Phase 69-3: Changed to BTreeSet for determinism
+                std::collections::BTreeSet::new()
             }
 
             /// Phase 26-A-4: ValueIdベースのパラメータ判定（Mock版）
@@ -1197,8 +1199,9 @@ mod tests {
             fn get_block_predecessors(
                 &self,
                 _block: BasicBlockId,
-            ) -> std::collections::HashSet<BasicBlockId> {
-                std::collections::HashSet::new()
+            ) -> std::collections::BTreeSet<BasicBlockId> {
+                // Phase 69-3: Changed to BTreeSet for determinism
+                std::collections::BTreeSet::new()
             }
 
             /// Phase 26-A-4: ValueIdベースのパラメータ判定（Mock版・パラメータなし）