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/lowering/update_env.rs

@@ -140,14 +140,36 @@ impl<'a> UpdateEnv<'a> {
     /// env.resolve("digit_pos") → env.get("digit_value") → Some(ValueId(X))
     /// ```
     pub fn resolve(&self, name: &str) -> Option<ValueId> {
-        // Phase 247-EX: Check if this is a promoted variable
-        if self.promoted_loopbodylocals.iter().any(|v| v == name) {
+        // Phase 247-EX: Check if this is a promoted variable (digit_pos) or its derived carrier name (digit_value)
+        let promoted_key = if self.promoted_loopbodylocals.iter().any(|v| v == name) {
+            Some(name.to_string())
+        } else if let Some(base) = name.strip_suffix("_value") {
+            let candidate = format!("{}_pos", base);
+            if self.promoted_loopbodylocals.iter().any(|v| v == &candidate) {
+                Some(candidate)
+            } else {
+                None
+            }
+        } else {
+            None
+        };
+
+        if let Some(promoted_name) = promoted_key {
+            // Prefer the freshly computed body-local value if it exists (digit_pos)
+            if let Some(val) = self.body_local_env.get(&promoted_name) {
+                eprintln!(
+                    "[update_env/phase247ex] Resolved promoted '{}' from body-local env: {:?}",
+                    name, val
+                );
+                return Some(val);
+            }
+
             // Phase 247-EX: Naming convention - "digit_pos" → "digit_value" (not "digit_pos_value")
             // Extract base name: "digit_pos" → "digit", "pos" → "pos"
-            let base_name = if name.ends_with("_pos") {
-                &name[..name.len() - 4]  // Remove "_pos" suffix
+            let base_name = if promoted_name.ends_with("_pos") {
+                &promoted_name[..promoted_name.len() - 4]  // Remove "_pos" suffix
             } else {
-                name
+                promoted_name.as_str()
             };
 
             // Priority 1a: Try <base>_value (integer carrier for NumberAccumulation)