## Summary

`gpui_macos` links two private CoreGraphics Services APIs
unconditionally:

```rust
#[link(name = "CoreGraphics", kind = "framework")]
unsafe extern "C" {
    fn CGSMainConnectionID() -> id;
    fn CGSSetWindowBackgroundBlurRadius(...) -> i32;
}
```

Apple rejects App Store submissions that reference private APIs, so any
application built on GPUI is ineligible for the Mac App Store —
regardless of
whether it ever sets `WindowBackgroundAppearance::Blurred`, since the
symbols are
linked either way.

Their only call site is the `NSAppKitVersionNumber <
NSAppKitVersionNumber12_0`
branch: macOS 11 and earlier. On macOS 12+ the same effect already goes
through
`NSVisualEffectView`, which the existing comment notes is the better
path anyway
(it downsamples the backdrop and gives more control over the effect
layer).

This deletes the legacy branch and the extern block.

## Changed from the previous revision of this PR

This originally added a `macos_app_store` feature gate. @reflectronic
suggested
deleting the legacy `WindowBackgroundAppearance::Blurred` support
instead of
carrying a flag — that is the better call and this PR now does that.

It is the stronger fix on the merits, not just the smaller one: deleting
makes
every GPUI consumer App Store eligible, whereas a feature flag only
helps those
who opt in and leaves the default build ineligible. It also removes a
conditional-compilation path rather than adding one. The diff went from
3 files / +13 −7 to **1 file / +23 −52**.

## Notes for review

Two details that are easy to miss:

- `this.native_window.windowNumber()` resolved through cocoa's
**`NSEvent`**
trait rather than `NSWindow` — `id` is untyped, so it picks up every
trait in
scope. Removing that call makes the `NSEvent` import genuinely unused,
so it is
dropped. Verified against a build of unmodified `main`, which does not
emit
  that warning, to confirm this is a consequence of the change and not
  pre-existing.
- `NSAppKitVersionNumber` and `NSAppKitVersionNumber12_0` had no other
users and
  are dropped with it.

The `NSVisualEffectView` branch is preserved verbatim; the only change
to it is
one level of de-indentation.

## Behaviour change

On macOS 11 and earlier, `WindowBackgroundAppearance::Blurred` no longer
applies
a blur — the window falls back to the `NSVisualEffectView` path, which
those
releases handle via `CAProxyLayer` rather than concrete sublayers. macOS
11 is
past end of life. Every other platform and macOS 12+ are unaffected.

## Verification

`cargo check -p gpui_macos` on macOS — clean, no errors and no new
warnings.

The identical change was also applied to a downstream GPUI fork and
built against
a ~60k-line application using `gpui-component`, where CI passed on
macOS, Ubuntu
and Windows.

Release Notes:

- N/A