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/basic_block.rs

@@ -6,7 +6,7 @@
 
 use super::{EffectMask, MirInstruction, SpannedInstRef, SpannedInstruction, ValueId};
 use crate::ast::Span;
-use std::collections::HashSet;
+use std::collections::BTreeSet; // Phase 69-3: HashSet → BTreeSet for determinism
 use std::fmt;
 
 /// Unique identifier for basic blocks within a function
@@ -60,10 +60,12 @@ pub struct BasicBlock {
     pub terminator_span: Option<Span>,
 
     /// Predecessors in the control flow graph
-    pub predecessors: HashSet<BasicBlockId>,
+    /// Phase 69-3: BTreeSet for deterministic iteration order
+    pub predecessors: BTreeSet<BasicBlockId>,
 
     /// Successors in the control flow graph
-    pub successors: HashSet<BasicBlockId>,
+    /// Phase 69-3: BTreeSet for deterministic iteration order
+    pub successors: BTreeSet<BasicBlockId>,
 
     /// Combined effect mask for all instructions in this block
     pub effects: EffectMask,
@@ -84,8 +86,8 @@ impl BasicBlock {
             instruction_spans: Vec::new(),
             terminator: None,
             terminator_span: None,
-            predecessors: HashSet::new(),
-            successors: HashSet::new(),
+            predecessors: BTreeSet::new(), // Phase 69-3: BTreeSet for determinism
+            successors: BTreeSet::new(),   // Phase 69-3: BTreeSet for determinism
             effects: EffectMask::PURE,
             reachable: false,
             sealed: false,
@@ -387,7 +389,8 @@ impl BasicBlock {
     }
 
     /// Check if this block dominates another block (simplified check)
-    pub fn dominates(&self, other: BasicBlockId, dominators: &[HashSet<BasicBlockId>]) -> bool {
+    /// Phase 69-3: Changed to BTreeSet for determinism
+    pub fn dominates(&self, other: BasicBlockId, dominators: &[BTreeSet<BasicBlockId>]) -> bool {
         if let Some(dom_set) = dominators.get(other.to_usize()) {
             dom_set.contains(&self.id)
         } else {