refactor(rag_engine): obliterate legacy _chunk_code wrapper (Phase 5)

Phase 5 (1 of 9 cruft sites obliterated):

OBLITERATED: RAGEngine._chunk_code wrapper. It delegated to _chunk_code_result
and provided a fallback to _chunk_text on AST failure.

Migration: index_file() now calls _chunk_code_result directly with .ok check
+ chunk-size threshold check + fallback to _chunk_text inline. The structured
ErrorInfo is propagated if needed (no caller currently consumes it).

Sub-track 5 tests updated:
- tests/tier2/phase13_invariant_test.py: _chunk_code moved to obliterated list
- tests/tier2/phase13_site2_test.py: _legacy_no_broad_except -> _legacy_obliterated
- tests/test_cruft_removal.py: 2 new tests (wrapper-obliterated invariant +
  caller-uses-result invariant)

PITFALL encountered: the edit_file tool removed a leading space on the
next class method's 'def' line, causing an IndentationError. Fixed by
binary-write replacement preserving CRLF + leading-space styleguide convention
(project uses 1-space indentation; class body methods start at column 1).

Test result: 124/124 pass.
Audit gate: src/rag_engine.py --strict exits 0 (no new violations).
Wrapper count: 3 -> 2 (Phase 6 remaining: gui_2 2).

tests/test_cruft_removal.py

@@ -120,4 +120,30 @@ def test_phase4_list_minimax_models_wrapper_obliterated():
     from src import ai_client
     assert not hasattr(ai_client, "_list_minimax_models"), (
         "_list_minimax_models legacy wrapper must be OBLITERATED."
+    )
+
+
+# ============ Phase 5 (rag_engine wrappers) ============
+
+def test_phase5_chunk_code_wrapper_obliterated():
+    """Phase 5 invariant: the legacy RAGEngine._chunk_code wrapper is DELETED."""
+    from src.rag_engine import RAGEngine
+    assert not hasattr(RAGEngine, "_chunk_code"), (
+        "RAGEngine._chunk_code legacy wrapper must be OBLITERATED."
+    )
+
+
+def test_phase5_chunk_code_caller_uses_result():
+    """The caller in index_file uses _chunk_code_result(...).ok directly."""
+    from src.rag_engine import RAGEngine
+    import inspect
+    src_text = inspect.getsource(RAGEngine.index_file)
+    assert "_chunk_code_result" in src_text, "caller must use _chunk_code_result"
+    # Check no bare _chunk_code( call (without _result suffix)
+    import re
+    bare_calls = re.findall(r"\b_chunk_code\(", src_text)
+    bare_calls = [c for c in bare_calls if c == "_chunk_code("]
+    assert len(bare_calls) == 0, (
+        f"caller should not call _chunk_code (legacy wrapper); "
+        f"found {len(bare_calls)} bare calls"
     )