refactor: tui.rs extract several pieces (#7461)

Pull FrameRequester out of tui.rs into its own module and make a
FrameScheduler struct. This is effectively an Actor/Handler approach
(see https://ryhl.io/blog/actors-with-tokio/). Adds tests and docs.

Small refactor of pending_viewport_area logic.

codex-rs/Cargo.lock

@@ -1615,6 +1615,7 @@ dependencies = [
  "textwrap 0.16.2",
  "tokio",
  "tokio-stream",
+ "tokio-util",
  "toml",
  "tracing",
  "tracing-appender",
@@ -6600,6 +6601,7 @@ dependencies = [
  "futures-sink",
  "futures-util",
  "pin-project-lite",
+ "slab",
  "tokio",
 ]