validation: select the frozen DFG manifest without reading the checkout

36ad523 made a manifest file missing from the checkout a 'Source mismatch',
which also broke the tests' manifest-selection helper: it reused
verify_source with hashing neutralised but still read the (renamed) files.
run_refined_dfg.source_manifest(collision, observation_mode) now owns the
frozen-manifest selection; verify_source uses it, then verifies the
checkout. The test helper calls source_manifest directly; source identity
stays owned by test_source_pin. Same errors, manifests and pins.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/validation/run_refined_dfg.py

@@ -281,16 +281,21 @@ def host_geometry(source, h, collision="SRT", jet_scenario="none", placement="wi
     return cfg, pair, body, host
 
 
-def verify_source(source, collision="SRT", observation_mode="last_base_step", jet_scenario="none", observe_cv=False):
+def source_manifest(collision="SRT", observation_mode="last_base_step"):
+    """The frozen historical manifest for this collision/observation mode (no checkout read)."""
     if collision not in ("SRT", "MRT"):
         raise ValueError("Matched collision must be SRT or MRT")
     if observation_mode not in ("last_base_step", "window"):
         raise ValueError("Unknown refined observation mode")
-    if observe_cv and jet_scenario == "none":
-        raise ValueError("--observe-cv requires an explicit public Jet scenario")
     name = ("source_manifest_window.json" if observation_mode == "window" else
             "source_manifest.json" if collision == "SRT" else "source_manifest_mrt.json")
-    expected = json.loads((HERE / "data/refined_dfg" / name).read_text())
+    return json.loads((HERE / "data/refined_dfg" / name).read_text())
+
+
+def verify_source(source, collision="SRT", observation_mode="last_base_step", jet_scenario="none", observe_cv=False):
+    expected = source_manifest(collision, observation_mode)
+    if observe_cv and jet_scenario == "none":
+        raise ValueError("--observe-cv requires an explicit public Jet scenario")
     if jet_scenario != "none":
         if collision != "MRT" or observation_mode != "window":
             raise ValueError("Public Jet scenarios require MRT and --exchange-window")