perf: fast-path identical snapshots in diff_snapshots (#922)

* Update agent-browser optimization plan: PR #916 submitted, Phase 2 complete

Co-authored-by: Hermes <agent@hermes.ai>

* perf(snapshot-diff): fast-path identical snapshots

Co-authored-by: Hermes <agent@hermes.ai>

* chore(pr): remove unrelated docs from snapshot-diff fast-path

---------

Co-authored-by: Merlin <merlin@rbeckner.com>
Co-authored-by: Hermes <agent@hermes.ai>

cli/src/native/diff.rs

@@ -101,6 +101,21 @@ pub fn diff_screenshot(
 
 /// Compute a snapshot diff using the Myers algorithm via the `similar` crate.
 pub fn diff_snapshots(before: &str, after: &str) -> SnapshotDiffResult {
+    // Fast path: identical inputs.
+    // This avoids constructing the `similar` TextDiff object and running the diff
+    // iteration when agents compare a snapshot to itself (common in retry/loop
+    // workloads).
+    if before == after {
+        let unchanged = before.lines().count();
+        return SnapshotDiffResult {
+            diff: String::new(),
+            additions: 0,
+            removals: 0,
+            unchanged,
+            changed: false,
+        };
+    }
+
     let text_diff = TextDiff::from_lines(before, after);
 
     let mut additions = 0usize;
@@ -192,4 +207,68 @@ mod tests {
         assert_eq!(result.unchanged, 1);
         assert!(!result.diff.is_empty());
     }
+
+    #[test]
+    fn test_diff_snapshots_identical_fast_path() {
+        let input = "hello\nworld\n";
+        let result = diff_snapshots(input, input);
+        assert!(!result.changed);
+        assert_eq!(result.additions, 0);
+        assert_eq!(result.removals, 0);
+        assert_eq!(result.unchanged, input.lines().count());
+        assert!(result.diff.is_empty());
+    }
+
+    #[test]
+    #[ignore]
+    fn bench_diff_snapshots_identical_and_changed() {
+        use std::hint::black_box;
+        use std::time::Instant;
+
+        let identical_a = (0..200)
+            .map(|i| format!("line {i}"))
+            .collect::<Vec<_>>()
+            .join("\n");
+        let identical_b = identical_a.clone();
+
+        let changed_a = identical_a.clone();
+        let changed_b = (0..200)
+            .map(|i| {
+                if i == 123 {
+                    format!("line {i} changed")
+                } else {
+                    format!("line {i}")
+                }
+            })
+            .collect::<Vec<_>>()
+            .join("\n");
+
+        // Keep the iteration count high enough to measure, but low enough
+        // to avoid long CI times when someone runs `--ignored`.
+        let iters = 50_000usize;
+
+        let start = Instant::now();
+        let mut acc_changed = 0usize;
+        for _ in 0..iters {
+            let r = diff_snapshots(black_box(&identical_a), black_box(&identical_b));
+            acc_changed ^= r.unchanged;
+        }
+        let identical_ms = start.elapsed().as_secs_f64() * 1000.0;
+
+        let start = Instant::now();
+        let mut acc_changed2 = 0usize;
+        for _ in 0..iters {
+            let r = diff_snapshots(black_box(&changed_a), black_box(&changed_b));
+            acc_changed2 ^= r.additions;
+        }
+        let changed_ms = start.elapsed().as_secs_f64() * 1000.0;
+
+        // Prevent the compiler from optimizing everything away.
+        black_box(acc_changed);
+        black_box(acc_changed2);
+
+        println!(
+            "bench_diff_snapshots_identical_and_changed: iters={iters} identical_ms={identical_ms:.2} changed_ms={changed_ms:.2}"
+        );
+    }
 }