refactor(rewrite): Phase 287 P5 - Code cleanup after toString SSOT

Three targeted cleanups following Phase 287 P4 toString normalization:

1. known.rs: Extract primitive guard into helper function
   - Unified 4 duplicate guard blocks (60 lines) into single function (15 lines)
   - 75% code reduction with improved testability
   - Function: should_block_primitive_str_rewrite()

2. special.rs: Extract trace helper
   - Unified 3 duplicate NYASH_STATIC_CALL_TRACE checks
   - Function: trace_tostring()

3. boxcall.py: Constant-ify magic numbers
   - UNIVERSAL_SLOT_TOSTRING = 0 (self-documenting)
   - TOSTRING_METHODS = ("toString", "stringify", "str")

Total impact: ~45 lines reduced, improved maintainability.

SSOT maintained: toString always uses universal slot #0.

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

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

src/llvm_py/instructions/boxcall.py

@@ -4,6 +4,10 @@ Core of Nyash's "Everything is Box" philosophy
 """
 
 import llvmlite.ir as ir
+
+# Phase 287 P5: Universal slot constants (SSOT)
+UNIVERSAL_SLOT_TOSTRING = 0  # toString/stringify/str (all types)
+TOSTRING_METHODS = ("toString", "stringify", "str")
 from typing import Dict, List, Optional, Any
 from instructions.safepoint import insert_automatic_safepoint
 from naming_helper import encode_static_method
@@ -93,7 +97,7 @@ def lower_boxcall(
 
     # Phase 287 P4: Universal slot #0 handling (toString/stringify/str)
     # SSOT: toString is ALWAYS slot #0, works on ALL types including primitives
-    if method_id == 0 and method_name in ("toString", "stringify", "str"):
+    if method_id == UNIVERSAL_SLOT_TOSTRING and method_name in TOSTRING_METHODS:
         import os, sys
         if os.environ.get('NYASH_LLVM_TRACE_SLOT') == '1':
             print(f"[llvm-py/slot] Universal slot #0: {method_name} on box_vid={box_vid}", file=sys.stderr)