fix(qt): MainThreadMarker comes from objc2-foundation in this generation

The Mac build caught it on the first try, which is exactly what it was for:
`use objc2::MainThreadMarker` does not resolve — in the objc2 0.5 /
objc2-foundation 0.2 generation the type is exported by FOUNDATION
(objc2-foundation-0.2.2/src/lib.rs:152, ungated); it only moved to the objc2
root later. The reference imports it from the same place
(crates/qbz/src/tray/macos.rs:38), which is where this should have been read
from in the first place rather than assumed from a newer API.

The direct `objc2` dependency goes with it: nothing here names a type from
that crate.

cc

crates/Cargo.lock

@@ -6624,8 +6624,8 @@ dependencies = [
  "ksni",
  "libc",
  "log",
- "objc2 0.5.2",
  "objc2-app-kit 0.2.2",
+ "objc2-foundation 0.2.2",
  "open",
  "qbz-app",
  "qbz-audio",

crates/qbz-qt/Cargo.toml

@@ -172,7 +172,13 @@ cxx-qt-build = "0.7"
 # behind cfg, so `cargo build` cannot see it. First real check is a macOS
 # build — the Mac mini (192.168.0.69) is where that happens.
 [target.'cfg(target_os = "macos")'.dependencies]
-objc2 = "0.5"
+# `MainThreadMarker` — in THIS generation it is exported by objc2-foundation,
+# not by the objc2 root (objc2-foundation-0.2.2/src/lib.rs:152, ungated). The
+# reference imports it from the same place. There is no direct `objc2`
+# dependency because nothing here names a type from it.
+objc2-foundation = { version = "0.2", default-features = false, features = [
+    "std",
+] }
 objc2-app-kit = { version = "0.2", default-features = false, features = [
     "std",
     # NSApplication: the class and `setActivationPolicy`.

crates/qbz-qt/src/tray_qt.rs

@@ -183,8 +183,12 @@ pub(crate) fn handle() -> Option<&'static TrayHandle> {
 /// reference's own `tray::set_mac_dock_hidden` (`tray/mod.rs:285-291`).
 #[cfg(target_os = "macos")]
 pub(crate) fn set_mac_dock_hidden(hidden: bool) {
-    use objc2::MainThreadMarker;
+    // `MainThreadMarker` lives in objc2-FOUNDATION in the 0.2/0.5 generation,
+    // not at the objc2 root — that is where it moved later. The reference
+    // imports it from exactly here (crates/qbz/src/tray/macos.rs:38), and a
+    // Mac build caught the wrong path on the first try.
     use objc2_app_kit::{NSApplication, NSApplicationActivationPolicy};
+    use objc2_foundation::MainThreadMarker;
 
     let Some(mtm) = MainThreadMarker::new() else {
         log::warn!("[tray] dock policy skipped: not on the AppKit main thread");