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.

src/causal.rs

@@ -582,6 +582,11 @@ mod inner {
     /// each virtual speedup and the change relative to that site's own 0%
     /// baseline. A near-zero column across speedups means: optimising this
     /// site buys you nothing — the RFC's headline answer.
+    ///
+    /// Ends with a one-line fidelity note (RFC 007 Validation): reported
+    /// impacts are conservative — the controller undershoots ideal injection
+    /// at high speedup pcts, so gains are lower bounds; site *rankings* are
+    /// unaffected.
     pub fn render_summary(results: &[ExperimentResult]) -> String {
         use std::fmt::Write;
         let mut s = String::new();
@@ -612,6 +617,12 @@ mod inner {
                 }
             }
         }
+        if !results.is_empty() {
+            let _ = writeln!(
+                s,
+                "note: impacts are lower bounds — undershoot grows with speedup pct; rankings unaffected"
+            );
+        }
         s
     }