feat(joinir): Phase 246-EX Part 2 - Exit PHI & step scheduling fixes

Phase 246-EX Part 2 completes the _atoi JoinIR integration:

Key fixes:
- Exit PHI connection: ExitLineReconnector now correctly uses exit PHI dsts
- Jump args preservation: BasicBlock.jump_args field stores JoinIR exit values
- instruction_rewriter: Reads jump_args, remaps JoinIR→HOST values by carrier order
- step_schedule.rs: New module for body-local init step ordering

Files changed:
- reconnector.rs: Exit PHI connection improvements
- instruction_rewriter.rs: Jump args reading & carrier value mapping
- loop_with_break_minimal.rs: Refactored step scheduling
- step_schedule.rs: NEW - Step ordering logic extracted

Tests: 931/931 PASS (no regression)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/join_ir_vm_bridge/joinir_block_converter.rs

@@ -348,6 +348,7 @@ impl JoinIrBlockConverter {
         args: &[ValueId],
         cond: &Option<ValueId>,
     ) -> Result<(), JoinIrVmBridgeError> {
+        // Phase 246-EX: Preserve ALL Jump args as metadata for exit PHI construction
         // Phase 27-shortterm S-4.4-A: Jump → Branch/Return
         debug_log!(
             "[joinir_block] Converting Jump args={:?}, cond={:?}",
@@ -376,9 +377,15 @@ impl JoinIrBlockConverter {
                     branch_terminator,
                 );
 
-                // Exit block
+                // Phase 246-EX: Store all Jump args in exit block metadata
+                // Exit block: Create with all Jump args stored as metadata
                 let exit_value = args.first().copied();
                 let mut exit_block = crate::mir::BasicBlock::new(exit_block_id);
+
+                // Phase 246-EX: Store Jump args in a new metadata field
+                // This preserves carrier values for exit PHI construction
+                exit_block.jump_args = Some(args.to_vec());
+
                 exit_block.terminator = Some(MirInstruction::Return { value: exit_value });
                 mir_func.blocks.insert(exit_block_id, exit_block);