feat(phase-5.3): Complete frontend cleanup - stop generating deprecated instructions

Phase 5.3 of MIR 35→26 reduction project:
- Replace RefNew with NewBox in MIR builder
- Remove unnecessary Const generation for box types
- Process arguments properly for NewBox instruction
- StringBox special handling maintained with NewBox

Frontend now generates only 26-instruction set compliant MIR.
Build successful with expected deprecation warnings.

local_tests/test_phase5_mir.nyash

@@ -0,0 +1,4 @@
+// Test that deprecated instructions are not generated
+local result = 42
+local text = new StringBox("Hello MIR")
+print(text.toString())