Summary

frontmost_bundle_id() is X11-only today, so on a Wayland session it returns
None for native windows and per-app profiles never fire (XWayland windows
aside). This adds two Wayland backends behind the existing selection, keeping
the X11 path as the universal fallback — no behavior change off Wayland, and
macOS/Windows untouched:

• wlroots — zwlr_foreign_toplevel_management_v1 (sway, Hyprland, river, Wayfire)
• GNOME Shell — a minimal, read-only companion extension that exports the
  focused window's WM_CLASS over D-Bus (Mutter offers no protocol/portal for this)

Implements the Wayland half of #95; complements the X11 backend from #122.

Backend selection

detect_session_kind() (XDG_SESSION_TYPE, falling back to
WAYLAND_DISPLAY/DISPLAY) sets the candidate order; the first that
initializes wins:

• Wayland → wlroots → GNOME extension → X11/XWayland
• X11 / unknown → X11

A candidate returns None when it can't initialize (wlr manager absent on
GNOME, extension not installed, ...), so unsupported compositors fall through to
X11 exactly as today. Selection is once-per-process; landing on X11 while on
Wayland logs a hint to install the extension.

Compositor coverage

Files

• src/linux/wlr_foreign_toplevel.rs — binds the foreign-toplevel manager,
  roundtrips per poll, returns the activated toplevel's app_id.
• src/linux/gnome_shell.rs — blocking zbus proxy onto org.openlogi.Frontmost,
  with a per-call timeout so a stalled Shell can't wedge the poll thread.
• gnome-shell-extension/openlogi-frontmost@openlogi.dev/ — the extension. Reads
  only global.display.focus_window.get_wm_class(); no titles, contents, input,
  or UI. Targets GNOME Shell 45–50.
• src/linux.rs — dispatch refactored to a FrontmostSource trait + ordered
  candidate list. New deps (wayland-client, wayland-protocols-wlr, zbus)
  under the existing cfg(target_os = "linux") target.

Identifier semantics — a design call I'd like your read on

GNOME and X11 both return WM_CLASS, so a profile created on X11 carries over to
GNOME/Wayland unchanged. wlroots returns the native xdg app_id, a different
namespace — and since profile lookup is an exact match, a profile created under
wlroots won't match one created under GNOME/X11. I return each compositor's
native identifier rather than a lossy WM_CLASS guess (stripping reverse-DNS and
re-capitalizing is wrong for many apps); reconciling the namespaces belongs in a
single normalization layer over frontmost_bundle_id(), which I left out to keep
this PR focused. Happy to add that pass, or to standardize on one identifier
across all Linux backends — which do you prefer?

Testing

Validated end-to-end on Ubuntu 26.04, GNOME Shell 50.1, Wayland, rustc 1.96:

• Extension State: ACTIVE; gdbus call ... GetFocusedWmClass returns and tracks
  the focused window's WM_CLASS.
• cargo run --example frontmost_app -p openlogi-hook follows focus live across
  native-Wayland apps (Ptyxis → org.gnome.Ptyxis, Nautilus → org.gnome.Nautilus)
  and Firefox (firefox_firefox) — windows the X11 backend reports as None.

Not yet hardware-tested: the wlroots backend. It compiles and follows the
protocol spec, but I don't run a wlroots compositor — a sanity check from a
sway/Hyprland user would be welcome, or I can spin one up before merge.

Install (GNOME)

UUID=openlogi-frontmost@openlogi.dev
DEST="$HOME/.local/share/gnome-shell/extensions/$UUID"
mkdir -p "$DEST"
cp crates/openlogi-hook/gnome-shell-extension/$UUID/{metadata.json,extension.js} "$DEST"/
# Wayland can't hot-reload the shell — log out/in, then:
gnome-extensions enable "$UUID"

Open questions

1. Extension UUID / D-Bus name use openlogi.* as placeholders — what namespace
   do you want? (constants mirrored in gnome_shell.rs.)
2. Extension distribution: bundle-and-document (current), ship to
   extensions.gnome.org, or auto-install from the app?

Checklist

• Linux-only (cfg(target_os = "linux")); macOS/Windows untouched.
• Falls through to X11 when no Wayland backend initializes.
• GNOME backend validated on real hardware (Ubuntu 26.04 / GNOME 50.1 / Wayland).
• wlroots backend validated on a wlroots compositor (help wanted).