feat(clients/windows): game library page + session window placement + help polish

UX polish batch 2 (plan workstreams D/E5/E6):

- D: a mouse/keyboard game library page over pf-client-core::library (the GTK
  ui_library.rs counterpart): responsive 2-6 column poster grid (2:3 portraits,
  store badge, monogram placeholder while art streams in), loading / error+retry
  / empty / grid states, tap-to-play via a normal spawn carrying --launch id.
  Poster bytes land in a disk cache (%LOCALAPPDATA%\punktfunk\art-cache) so the
  Image widget loads file:/// URIs (reactor has no from-bytes source) and
  revisits render instantly. Reached from a paired host's "Browse library..."
  menu item behind the new "Show game library (experimental)" Settings toggle
  (the shared library_enabled field, Apple/GTK parity). A Shared::library_gen
  generation guard keeps a superseded fetch from publishing (speed-test pattern).

- E5: the session window opens at the shell's own top-left (--window-pos, new
  SessionOpts::window_pos) instead of centered on the primary display - streams
  land on the monitor the shell is on, and the hide/restore handoff reads as one
  window changing content. Fullscreen follows that display.

- E6: the Help shortcuts add Alt+Enter and the controller escape chord
  (LB+RB+Start+Back, hold to disconnect).

Also: rustfmt normalization of the merged probe helper.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-client-core/src/trust.rs

@@ -246,11 +246,12 @@ pub fn probe_reachable_many(
 ) -> Vec<bool> {
     let handles: Vec<_> = targets
         .into_iter()
-        .map(|(addr, port)| {
-            std::thread::spawn(move || NativeClient::probe(&addr, port, timeout))
-        })
+        .map(|(addr, port)| std::thread::spawn(move || NativeClient::probe(&addr, port, timeout)))
         .collect();
-    handles.into_iter().map(|h| h.join().unwrap_or(false)).collect()
+    handles
+        .into_iter()
+        .map(|h| h.join().unwrap_or(false))
+        .collect()
 }
 
 /// App settings, persisted as JSON. Stringly-typed gamepad/compositor prefs so the file

crates/pf-presenter/src/run.rs

@@ -31,6 +31,12 @@ pub struct SessionOpts {
     pub window_title: String,
     /// Start fullscreen (gamescope / `--fullscreen`).
     pub fullscreen: bool,
+    /// The window's top-left in desktop coordinates; `None` = centered on the primary
+    /// display. The shells pass their own window's position so the stream opens on the
+    /// SAME monitor (and the shell⇄session visibility handoff reads as one window
+    /// changing content, not a window jumping displays). Fullscreen follows the display
+    /// this lands on.
+    pub window_pos: Option<(i32, i32)>,
     /// Print `stats:` lines (Ctrl+Alt+Shift+S toggles live).
     pub print_stats: bool,
     /// Emit the `{"ready":true}` stdout line after the first presented frame.
@@ -239,7 +245,11 @@ fn run_inner(mut opts: SessionOpts, mut mode: ModeCtl) -> Result<Option<Outcome>
         .map_err(|e| anyhow::anyhow!("register FrameWake event: {e}"))?;
     let mut window = {
         let mut b = video.window(&opts.window_title, 1280, 720);
-        b.position_centered().resizable().vulkan();
+        match opts.window_pos {
+            Some((x, y)) => b.position(x, y),
+            None => b.position_centered(),
+        };
+        b.resizable().vulkan();
         if opts.fullscreen {
             b.fullscreen();
         }