fix(display): retain provider provenance for persisted fallback readings

agent/context_breakdown.py

@@ -97,7 +97,7 @@ def context_display_source(compressor: Any) -> str:
     """
     real = getattr(compressor, "last_real_prompt_tokens", None)
     shown = getattr(compressor, "last_prompt_tokens", 0) or 0
-    return "local_estimate" if isinstance(real, (int, float)) and shown != real else "provider_usage"
+    return "local_estimate" if isinstance(real, (int, float)) and shown > 0 and shown != real else "provider_usage"
 
 
 def context_usage_fields(compressor: Any) -> Dict[str, Any]:

tests/agent/test_context_display_provenance.py

@@ -36,3 +36,8 @@ def test_preflight_seed_does_not_label_actual_usage_estimated():
     comp.update_from_response({"prompt_tokens": 1234, "completion_tokens": 20})
     assert _get_usage(agent).get("context_estimated") is False
     assert _get_usage(agent)["context_used"] == 1234
+    comp.last_prompt_tokens = -1
+    assert "context_used" not in _get_usage(agent)
+    from agent.context_breakdown import context_display_source
+    # A cleared live gauge must not re-label a persisted provider fallback.
+    assert context_display_source(comp) == "provider_usage"