refactor: replace AppProxy poll loop with MainThreadPoster

[BumpyClock]

Motivation

AppProxy scheduled cross-thread work onto the main thread by draining an
std::mpsc channel from a foreground task that woke every 16ms via a
background timer. The loop was flagged as a wart in the app-platform work: an
idle tray app pays a fixed wakeup cost forever, purely because the fork had no
dependency-free way to wake the main thread from a Send context. That is
exactly the battery profile a tray-first app is supposed to avoid.

The swap

The fork now exposes the primitive (gpui#7): App::main_thread_poster() hands
back a Clone + Send + Sync MainThreadPoster whose post() enqueues a
closure and wakes the run loop through the platform dispatcher — no polling, an
idle app stays parked.

• AppProxy is now backed by the poster; the timer/drain loop and its
  gpui::Task (_drain_task) are deleted.
• The shutdown boundary stays ours: a closed: AtomicBool, set at
  ShutdownRequested (which precedes app teardown), is checked in dispatch
  and re-checked inside every posted closure on the main thread. Because
  close() also runs on the main thread, any closure the pump dispatches after
  close() drops its payload — preserving the old drain loop's
  discard-after-close contract. The poster's own after-teardown false is a
  second layer, not a replacement.
• The two old unit tests were locked to the mpsc channel internals; replaced
  with three #[gpui::test] behavior tests (runs-until-closed,
  queued-work-discarded-after-close, callback-shutdown-discards-following-work)
  driving a real &mut App. Adds gpui test-support to the crate's
  dev-dependencies (already used by crates/ui).

Pinned SHA

gpui / gpui_platform rev and the vendor/gpui submodule are pinned to
2ee4fa6587f9fc8eef012f86897034cb7d859d9d (main + 1, additive). The workspace
version and COMPATIBILITY.md bump are intentionally not done here — that
happens at merge time via /update-gpui discipline.

Merge preconditions

1. gpui#7 merges → re-pin gpui/gpui_platform + submodule to the merge SHA
   via /update-gpui.
2. PR#8 merges → rebase this branch onto the updated feat/app-platform.

Gates

• cargo test -p gpui-component-app — 49 unit + headless + doc, all green
• cargo clippy --workspace --all-targets -- --deny warnings — clean
• cargo fmt --check — clean
• app_shell_tray --smoke and app_shell --smoke — both exit 0

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: e65abb2e-b23e-47c3-aa5d-db1a63ab24fb

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/appproxy-poster

---

_{Comment @coderabbitai help to get the list of available commands.}