feat(#000057): --skip-solo flag — arborist-only sweeps without redundant solo data

fox 2026-05-19: 'we don't need to redo anything'. The arborist+qwen-
nothink sweep would re-run qwen-nothink solo on 1158 records that
phase 1 already produced (control_sweep_2026-05-19T21-52-56Z.jsonl,
calibrated rescore on disk). Wasteful — qwen-nothink solo behaviour
doesn't depend on retrieval being on, so phase 1's numbers stand.

--skip-solo runs ONLY the arborist arm. Implementation: skip the
per-variant model loop in _process_item when the flag is set; the
arborist arm below it still runs if arborist_on. The spend banner
zeroes the solo-call count so the operator sees the actual LLM
budget for the arborist-only run.

Example: a full arborist+qwen-nothink sweep against the 386-item
fixture goes from 2316 LLM calls (1158 solo + 1158 arborist) to
1158 calls (arborist only). Halves wall-clock on single-worker
llama.cpp.

pytest 27/27, imports clean, --help shows the flag.

bench/control_sweep.py

@@ -134,13 +134,18 @@ VARIANTS = {
 def _process_item(idx: int, it: dict, variants: list[str],
                   models: list[str], shards_dir: Path,
                   arborist_on: bool, arborist_ref: str,
-                  ts: str, judge_fn=None) -> list[dict]:
+                  ts: str, judge_fn=None,
+                  skip_solo: bool = False) -> list[dict]:
     """All variants × models for ONE fixture item. Self-contained:
     its own qa_db, its own clients — safe to run concurrently.
 
     ``judge_fn`` defaults to the code judge for back-compat with any
     direct callers that pre-date the --judge switch; main() passes
-    the user's choice explicitly."""
+    the user's choice explicitly. ``skip_solo`` lets a caller run
+    ONLY the arborist arm — useful when the solo data for the same
+    model + fixture already exists in a prior sweep and the new run
+    is just adding the arborist arm (fox 2026-05-19: 'we don't need
+    to redo anything')."""
     if judge_fn is None:
         judge_fn = _judge_code.judge
     from arborist.qa.client import OpenAICompatibleClient
@@ -166,7 +171,10 @@ def _process_item(idx: int, it: dict, variants: list[str],
     try:
         for variant in variants:
             q_asked = VARIANTS[variant](q0)
-            for mkey in models:
+            # Solo arm is skipped when --skip-solo is set OR when the
+            # caller passed no models. Either way the arborist arm
+            # below still runs if arborist_on.
+            for mkey in [] if skip_solo else models:
                 cfg = MODELS[mkey]
                 try:
                     ans = cl(mkey).chat_completion(
@@ -382,6 +390,11 @@ def main() -> int:
                          "the original headless Opus judge (requires "
                          "ARBORIST_JUDGE_ENABLE=1 — gated to prevent "
                          "accidental quota burn).")
+    ap.add_argument("--skip-solo", action="store_true",
+                    help="skip the solo arm; run ONLY the arborist arm. "
+                         "Use when the model's solo data already exists "
+                         "from a prior sweep on the same fixture — avoids "
+                         "redundant LLM spend.")
     a = ap.parse_args()
     judge_fn, judge_self_test, judge_model_id = _JUDGES[a.judge]
 
@@ -397,7 +410,7 @@ def main() -> int:
     variants = [v for v in a.variants.split(",") if v in VARIANTS]
     items = json.loads(Path(a.fixture).read_text())[:a.n]
 
-    n_solo = len(models) * len(variants) * len(items)
+    n_solo = 0 if a.skip_solo else len(models) * len(variants) * len(items)
     arb_units = min(a.arborist_n, len(items))
     n_arb = len(variants) * arb_units
     print(f"#000057 control sweep — fixture={a.fixture}")
@@ -454,7 +467,7 @@ def main() -> int:
         futs = {
             ex.submit(_process_item, i, it, variants, models,
                       shards_dir, i <= arb_units, a.arborist_ref,
-                      ts, judge_fn): i
+                      ts, judge_fn, a.skip_solo): i
             for i, it in enumerate(items, 1)
             if i not in skip_items
         }