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/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -5,6 +5,7 @@ use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use super::super::trace;
 use crate::mir::loop_pattern_detection::error_messages;
+use crate::mir::join_ir::lowering::carrier_info::CarrierInit;
 
 /// Phase 185-2: Collect body-local variable declarations from loop body
 ///
@@ -511,6 +512,7 @@ impl MirBuilder {
             carrier_updates.contains_key(&carrier.name)
                 || carrier.role == CarrierRole::ConditionOnly
                 || carrier.init == CarrierInit::FromHost  // Phase 247-EX
+                || carrier.init == CarrierInit::LoopLocalZero // Phase 247-EX: Derived carrier (digit_value)
         });
 
         eprintln!(
@@ -524,18 +526,28 @@ impl MirBuilder {
         // need to be added to ConditionEnv with their initial values.
         for carrier in &carrier_info.carriers {
             if env.get(&carrier.name).is_none() {
-                // Allocate a new JoinIR ValueId for this carrier
-                let join_value = alloc_join_value();
+                // Use the carrier's assigned param ID when available to keep IDs aligned
+                let join_value = carrier
+                    .join_id
+                    .unwrap_or_else(|| alloc_join_value());
 
                 // Add to ConditionEnv
                 env.insert(carrier.name.clone(), join_value);
 
                 // Add to condition_bindings for later processing
-                condition_bindings.push(ConditionBinding {
-                    name: carrier.name.clone(),
-                    host_value: carrier.host_id,
-                    join_value,
-                });
+                // Loop-local carriers (e.g., digit_value) have no host slot; skip binding to avoid bogus copies.
+                if carrier.init != CarrierInit::LoopLocalZero {
+                    condition_bindings.push(ConditionBinding {
+                        name: carrier.name.clone(),
+                        host_value: carrier.host_id,
+                        join_value,
+                    });
+                } else {
+                    eprintln!(
+                        "[cf_loop/pattern2] Phase 247-EX: Skipping host binding for loop-local carrier '{}' (init=LoopLocalZero)",
+                        carrier.name
+                    );
+                }
 
                 eprintln!(
                     "[cf_loop/pattern2] Phase 176-5: Added body-only carrier '{}' to ConditionEnv: HOST {:?} → JoinIR {:?}",