Address review of two-stop branch

- lca._prefix_profile: anchor the prefix window to the prefix's own first arrival
  so the A-rate is measured over the prefix span (matches the design intent;
  no-op for the 0-based canonical pipeline).
- cli study tune: label file-originated stops as file_proposal rather than
  llm_after_veto_budget (the veto never applies to file proposals).
- spec.AdaptiveStopSpec: reject stable_checks > max_checks (would make
  convergence undetectable and silently disable Stop-A).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aituner/cli.py

@@ -376,6 +376,8 @@ def cmd_study_tune(args: argparse.Namespace) -> int:
                     "stop_authorized_by": (
                         "validator"
                         if (is_harness_stop or authorized)
+                        else "file_proposal"
+                        if proposal_source is not None
                         else "llm_after_veto_budget"
                     ),
                     "diagnosis": proposal.diagnosis,

src/aituner/lca.py

@@ -373,12 +373,13 @@ def _prefix_profile(
     length_mode: str,
 ) -> WorkloadProfile:
     prefix = requests[:index]
+    start = float(prefix[0].arrival_s) if prefix else float(window.window_start)
     end = float(prefix[-1].arrival_s) if prefix else float(window.window_start)
     prefix_window = WindowRecord(
         window_id=window.window_id,
         trace_path=window.trace_path,
         trace_type=window.trace_type,
-        window_start=window.window_start,
+        window_start=start,
         window_end=end,
         source_payload=window.source_payload,
     )

src/aituner/spec.py

@@ -364,6 +364,11 @@ class AdaptiveStopSpec:
             raise SpecError(
                 "trace.adaptive_stop.stable_checks and max_checks must be > 0."
             )
+        if stable_checks > max_checks:
+            raise SpecError(
+                "trace.adaptive_stop.stable_checks must be <= max_checks, "
+                "otherwise convergence can never be detected."
+            )
         return cls(
             enabled=enabled,
             tau=tau,