Integrate / Merge / Copy between branches

- Tools -> Integrate / Merge / Copy: pick op + source/target paths, runs into a
  new pending changelist; resolve (if needed) via the Resolve UI and submit
- Backend p4 merge/copy/integrate -c

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -1783,6 +1783,28 @@ async fn p4_resolve_file(state: State<'_, AppState>, file: String, mode: String)
     }
 }
 
+/// Propagate changes between branches/streams: op is "merge" (with resolve),
+/// "copy" (overwrite target), or "integrate" (classic). Opens the affected files
+/// into a fresh numbered changelist; the user then resolves (if needed) and
+/// submits via the normal pending-changelist flow. Returns the changelist number.
+#[tauri::command]
+async fn p4_branch_op(state: State<'_, AppState>, op: String, source: String, target: String) -> Result<String, String> {
+    let conn = current(&state)?;
+    let cmd = match op.as_str() {
+        "merge" => "merge",
+        "copy" => "copy",
+        _ => "integrate",
+    };
+    let src = source.trim();
+    let tgt = target.trim();
+    if src.is_empty() || tgt.is_empty() {
+        return Err("Source and target paths are required".into());
+    }
+    let cl = create_changelist(&conn, &format!("{cmd} {src} -> {tgt}"))?;
+    let out = run_text(&conn, &[cmd, "-c", &cl, src, tgt]).unwrap_or_default();
+    Ok(format!("CL {cl}\n{}", out.trim()))
+}
+
 /// Unified diff between two exact revisions (`p4 diff2 -du`).
 #[tauri::command]
 async fn p4_diff2(state: State<'_, AppState>, spec1: String, spec2: String) -> Result<String, String> {
@@ -2084,6 +2106,7 @@ pub fn run() {
             p4_sync_to,
             p4_labels,
             p4_label_tag,
+            p4_branch_op,
             p4_client_spec,
             p4_client_save,
             uasset_thumbnail,