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/runner/json_v0_bridge/lowering/loop_.rs

@@ -81,7 +81,8 @@ impl LoopFormOps for LoopFormJsonOps<'_> {
     fn get_block_predecessors(
         &self,
         block: BasicBlockId,
-    ) -> std::collections::HashSet<BasicBlockId> {
+    ) -> std::collections::BTreeSet<BasicBlockId> {
+        // Phase 69-3: Changed to BTreeSet for determinism
         self.f
             .blocks
             .get(&block)

src/runner/json_v1_bridge.rs

@@ -273,7 +273,7 @@ pub fn try_parse_v1_to_module(json: &str) -> Result<Option<MirModule>, String> {
                         block_ref.add_instruction(MirInstruction::Phi {
                             dst: ValueId::new(dst),
                             inputs: pairs,
-                type_hint: None, // Phase 63-6
+                            type_hint: None, // Phase 63-6
                         });
                         max_value_id = max_value_id.max(dst + 1);
                     }

src/runner/mir_json_v0.rs

@@ -179,7 +179,7 @@ pub fn parse_mir_v0_to_module(json: &str) -> Result<MirModule, String> {
                         block_ref.add_instruction(MirInstruction::Phi {
                             dst: ValueId::new(dst),
                             inputs: pairs,
-                type_hint: None, // Phase 63-6
+                            type_hint: None, // Phase 63-6
                         });
                         max_value_id = max_value_id.max(dst + 1);
                     }