Feature-gate the muda + gtk-rs cascade on Linux

[mxaddict]

Context

On Linux, floem unconditionally pulls in muda with the gtk feature:

# floem/Cargo.toml
[target.'cfg(any(target_os = "linux"))'.dependencies]
muda = { workspace = true, default-features = false, features = ["gtk"] }

muda's gtk feature drags in the whole gtk-rs sys cascade —
gtk-sys, gdk-sys, gdk-pixbuf-sys, pango-sys, atk-sys,
glib-sys, gobject-sys, plus gettext-sys. Each of those has a
build script that calls pkg-config against the host's GTK3 dev
files.

This is the right default for floem apps that want native menu bars
(global app menu on KDE / Unity, in-window menus on GNOME). Layer-shell
overlays, tray apps, single-window utilities — nothing menu-shaped —
inherit the whole cascade just for the link.

Concrete cost

• Cross-compile pain. Cross-target builds for
  aarch64-unknown-linux-gnu need an arm64 sysroot with GTK3 dev
  files (Ubuntu's :arm64 multiarch off ports.ubuntu.com, since
  security.ubuntu.com doesn't serve arm64 indexes). Musl targets
  need a hand-rolled glib sysroot.
• Cargo-deny noise. The gtk-rs GTK3 bindings family is marked
  unmaintained upstream (RUSTSEC-2024-0412/0413/0415/0416/0418/0419/0420),
  so every downstream pikr-style consumer has to ignore those
  advisories with rationale.
• Compile time. ~30 extra crates in the dep graph for a feature
  the consumer may not use.

Ask

Add a feature flag that gates the muda import (or at least the
gtk muda feature) — something like:

[features]
default = ["menus"]
menus = []

# under [target.'cfg(any(target_os = "linux"))'.dependencies]
muda = { workspace = true, default-features = false, features = ["gtk"], optional = true }

Then consumers that don't render menu bars can opt out:

[dependencies]
floem = { version = "...", default-features = false }

Keeping menus on by default preserves existing behaviour for
everyone else.

Real-world consumer

pikr (https://github.com/kryptic-sh/pikr) — a wlr-layer-shell
launcher built on floem 0.2 (and currently the
mxaddict/floem:layer-shell-port branch on top of current main).
Pikr never opens a menu bar but still inherits the gtk-rs cascade,
which (a) dropped aarch64-unknown-linux-gnu and
x86_64-unknown-linux-musl from its release matrix during the
floem-main migration and (b) requires a cargo-deny ignore list
for the unmaintained gtk-rs advisories.

I'm happy to draft the PR if the feature shape sounds reasonable.

[mxaddict]

Draft PR up: #1077. Single commit, menus-off build produces zero muda/gtk/glib-sys artifacts; menus-on (default) is unchanged. Happy to take review notes.