refactor(joinir): Phase 86 - Carrier init builder, debug migration, error tags

P1: Carrier Initialization Builder (HIGH) ✅
- New module: carrier_init_builder.rs (197 lines, 8 tests)
- Refactored loop_header_phi_builder.rs (-34 lines)
- Centralized CarrierInit value generation (SSOT)
- Eliminates scattered match patterns across header PHI, exit line
- Consistent debug output: [carrier_init_builder] format
- Net: -34 lines of duplicated logic

P2: Remaining DebugOutputBox Migration (QUICK) ✅
- Migrated carrier_info.rs::record_promoted_binding()
- Uses DebugOutputBox for JOINIR_DEBUG checks
- Maintains JOINIR_TEST_DEBUG override for test diagnostics
- Consistent log formatting: [context/category] message
- Net: +3 lines (SSOT migration)

P3: Error Message Centralization (LOW) ✅
- New module: error_tags.rs (136 lines, 5 tests)
- Migrated 3 error sites:
  * ownership/relay:runtime_unsupported (plan_validator.rs)
  * joinir/freeze (control_flow/mod.rs)
  * (ExitLine errors were debug messages, not returns)
- Centralized error tag generation (freeze, exit_line_contract, ownership_relay_unsupported, etc.)
- Net: +133 lines (SSOT module + tests)

Total changes:
- New files: carrier_init_builder.rs (197), error_tags.rs (136)
- Modified: 6 files
- Production code: +162 lines (SSOT investment)
- Tests: 987/987 PASS (982→987, +5 new tests)
- Phase 81 ExitLine: 2/2 PASS
- Zero compilation errors/warnings

Benefits:
✅ Single Responsibility: Each helper has one concern
✅ Testability: 13 new unit tests (8 carrier init, 5 error tags)
✅ Consistency: Uniform debug/error formatting
✅ SSOT: Centralized CarrierInit and error tag generation
✅ Discoverability: Easy to find all error types

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

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

src/mir/builder/control_flow/mod.rs

@@ -110,12 +110,13 @@ impl super::MirBuilder {
 
         // Phase 186: LoopBuilder Hard Freeze - Legacy path disabled
         // Phase 187-2: LoopBuilder module removed - all loops must use JoinIR
-        return Err(format!(
-            "[joinir/freeze] Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder has been removed.\n\
+        use crate::mir::join_ir::lowering::error_tags;
+        return Err(error_tags::freeze(&format!(
+            "Loop lowering failed: JoinIR does not support this pattern, and LoopBuilder has been removed.\n\
              Function: {}\n\
              Hint: This loop pattern is not supported. All loops must use JoinIR lowering.",
             self.current_function.as_ref().map(|f| f.signature.name.as_str()).unwrap_or("<unknown>")
-        ));
+        )));
     }
 
     /// Phase 49: Try JoinIR Frontend for mainline integration