Streams: list / switch / merge down / copy up

- Tools -> Streams: list server streams, switch the workspace to a stream,
  merge down from parent / copy up to parent (into a pending changelist)
- Backend p4 streams, client -s -S, merge/copy -S

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

src/p4.ts

@@ -68,6 +68,9 @@ export const p4 = {
   labels: () => invoke<{ label?: string; Update?: string; Owner?: string; Description?: string; [k: string]: unknown }[]>("p4_labels"),
   labelTag: (label: string, scope: string, rev = "") => invoke<string>("p4_label_tag", { label, scope, rev }),
   branchOp: (op: "merge" | "copy" | "integrate", source: string, target: string) => invoke<string>("p4_branch_op", { op, source, target }),
+  streams: () => invoke<{ Stream?: string; Name?: string; Type?: string; Parent?: string; [k: string]: unknown }[]>("p4_streams"),
+  switchStream: (stream: string) => invoke<P4Info>("p4_switch_stream", { stream }),
+  streamInteg: (stream: string, direction: "down" | "up") => invoke<string>("p4_stream_integ", { stream, direction }),
   diff: (file: string) => invoke<string>("p4_diff", { file }),
   revert: (files: string[]) => invoke<string>("p4_revert", { files }),
   submit: (description: string, files: string[]) =>