feat(joinir): Phase 224 - DigitPosPromoter for A-4 pattern (core complete)

- New DigitPosPromoter Box for cascading indexOf pattern detection
- Two-tier promotion strategy: A-3 Trim → A-4 DigitPos fallback
- Unit tests 6/6 PASS (comparison operators, cascading dependency)
- Promotion verified: digit_pos → is_digit_pos carrier

⚠️ Lowerer integration gap: lower_loop_with_break_minimal doesn't
recognize promoted variables yet (Phase 224-continuation)

🤖 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/pattern4_with_continue.rs

@@ -35,6 +35,7 @@ use crate::ast::ASTNode;
 use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use super::super::trace;
+use crate::mir::loop_pattern_detection::error_messages;
 
 /// Phase 194+: Detection function for Pattern 4
 ///
@@ -279,9 +280,8 @@ impl MirBuilder {
                                 );
                                 // Continue with Pattern4 lowering (fall through)
                             } else {
-                                return Err(format!(
-                                    "[cf_loop/pattern4] Trim pattern detected but not safe: carrier='{}', whitespace_count={}",
-                                    helper.carrier_name,
+                                return Err(error_messages::format_error_pattern4_trim_not_safe(
+                                    &helper.carrier_name,
                                     helper.whitespace_count()
                                 ));
                             }
@@ -290,10 +290,7 @@ impl MirBuilder {
                     }
                     ConditionPromotionResult::CannotPromote { reason, vars } => {
                         // Fail-Fast on promotion failure
-                        return Err(format!(
-                            "[cf_loop/pattern4] Cannot promote LoopBodyLocal variables {:?}: {}",
-                            vars, reason
-                        ));
+                        return Err(error_messages::format_error_pattern4_promotion_failed(&vars, &reason));
                     }
                 }
             }
@@ -316,7 +313,7 @@ impl MirBuilder {
             Err(e) => {
                 // Phase 195: Use unified trace
                 trace::trace().debug("pattern4", &format!("Pattern 4 lowerer failed: {}", e));
-                return Err(format!("[cf_loop/pattern4] Lowering failed: {}", e));
+                return Err(error_messages::format_error_pattern4_lowering_failed(&e));
             }
         };
 
@@ -344,10 +341,7 @@ impl MirBuilder {
         // (ExitMetaCollector silently skips missing carriers, but Pattern 4 expects all)
         for carrier in &carrier_info.carriers {
             if !exit_bindings.iter().any(|b| b.carrier_name == carrier.name) {
-                return Err(format!(
-                    "[cf_loop/pattern4] Carrier '{}' not found in variable_map - exit binding missing",
-                    carrier.name
-                ));
+                return Err(error_messages::format_error_pattern4_carrier_not_found(&carrier.name));
             }
         }