feat(#14): default bench_results_dir + testability seam

- sft.py: _DEFAULT_BENCH_RESULTS_DIR set to circuitforge-orch bench
  results path; set_default_bench_results_dir() seam for test isolation
- test fixture resets default to tmp_path to avoid real-fs interference
- 136 tests passing

Closes #14

app/sft.py

@@ -51,17 +51,26 @@ def _config_file() -> Path:
     return _ROOT / "config" / "label_tool.yaml"
 
 
+_DEFAULT_BENCH_RESULTS_DIR = "/Library/Development/CircuitForge/circuitforge-orch/scripts/bench_results"
+
+
+def set_default_bench_results_dir(path: str) -> None:
+    """Override the default bench_results_dir — used by tests to avoid real filesystem."""
+    global _DEFAULT_BENCH_RESULTS_DIR
+    _DEFAULT_BENCH_RESULTS_DIR = path
+
+
 def _get_bench_results_dir() -> Path:
     f = _config_file()
-    if not f.exists():
+    if f.exists():
-        return Path("/nonexistent-bench-results")
         try:
             raw = yaml.safe_load(f.read_text(encoding="utf-8")) or {}
+            d = raw.get("sft", {}).get("bench_results_dir", "")
+            if d:
+                return Path(d)
         except yaml.YAMLError as exc:
             logger.warning("Failed to parse SFT config %s: %s", f, exc)
-        return Path("/nonexistent-bench-results")
+    return Path(_DEFAULT_BENCH_RESULTS_DIR)
-    d = raw.get("sft", {}).get("bench_results_dir", "")
-    return Path(d) if d else Path("/nonexistent-bench-results")
 
 
 def _candidates_file() -> Path:

tests/test_sft.py

@@ -10,11 +10,14 @@ def reset_sft_globals(tmp_path):
     from app import sft as sft_module
     _prev_data    = sft_module._SFT_DATA_DIR
     _prev_cfg     = sft_module._SFT_CONFIG_DIR
+    _prev_default = sft_module._DEFAULT_BENCH_RESULTS_DIR
     sft_module.set_sft_data_dir(tmp_path)
     sft_module.set_sft_config_dir(tmp_path)
+    sft_module.set_default_bench_results_dir(str(tmp_path / "bench_results"))
     yield
     sft_module.set_sft_data_dir(_prev_data)
     sft_module.set_sft_config_dir(_prev_cfg)
+    sft_module.set_default_bench_results_dir(_prev_default)
 
 
 @pytest.fixture