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()