feat(causal): fidelity footer in render_summary

One unconditional footer line whenever there are results:
"note: impacts are lower bounds — undershoot grows with speedup pct;
rankings unaffected" — surfacing the RFC 007 Validation fidelity statement
where users actually look, instead of only in the RFC. Wording pinned by
the summary test.

tests/causal.rs

@@ -330,6 +330,10 @@ fn controller_produces_report() {
         let summary = smarm::causal::render_summary(&results);
         assert!(summary.contains("stage-x"), "summary: {summary}");
         assert!(summary.contains("items"), "summary: {summary}");
+        assert!(
+            summary.contains("impacts are lower bounds"),
+            "fidelity footer missing: {summary}"
+        );
 
         let coz = smarm::causal::render_coz(&results);
         assert!(coz.contains("experiment\tselected=stage-x"), "coz: {coz}");