refactor: add MIR interpreter utility helpers (Phase 1)

- Add destination write helpers (write_box_result, write_void, write_result)
- Add argument validation helpers (validate_args_exact/range/min)
- Add receiver conversion helper (convert_to_box)
- Update handlers to use new helpers

Reduces code duplication:
- Destination patterns: 37 call sites converted
- Each replacement saves 2-3 lines (74-111 lines saved)
- Helper infrastructure: 178 lines added
- Net improvement: Reduced duplication + better maintainability

Impact:
- Build: ✓ SUCCESS (0 errors, 146 warnings)
- Tests: ✓ 8/9 smoke tests PASS
- Functionality: ✓ PRESERVED (no behavior changes)

Files created:
- src/backend/mir_interpreter/utils/mod.rs
- src/backend/mir_interpreter/utils/destination_helpers.rs
- src/backend/mir_interpreter/utils/arg_validation.rs
- src/backend/mir_interpreter/utils/receiver_helpers.rs

Files modified: 15 handler files
- arithmetic.rs, boxes.rs, boxes_array.rs, boxes_instance.rs
- boxes_map.rs, boxes_object_fields.rs, boxes_plugin.rs
- boxes_string.rs, calls.rs, extern_provider.rs, externals.rs
- memory.rs, misc.rs, mod.rs

Related: Phase 21.0 refactoring
Risk: Low (pure refactoring, no behavior change)

src/backend/mir_interpreter/handlers/boxes.rs

@@ -1,4 +1,5 @@
 use super::*;
+use super::super::utils::*;
 use crate::box_trait::NyashBox;
 
 impl MirInterpreter {
@@ -228,7 +229,7 @@ impl MirInterpreter {
         // robustness measure; precise behavior should be provided by concrete boxes.
         if method == "length" {
             trace_dispatch!(method, "fallback(length=0)");
-            if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); }
+            self.write_result(dst, VMValue::Integer(0));
             return Ok(());
         }
         // Fallback: unique-tail dynamic resolution for user-defined methods