X11 backend: right-click titlebar window menu, matching Wayland's own

Closes the one real gap an X11/Wayland feature-parity audit found this
session (desktop icons, window-position memory, and static exclusive-zone
reservation were already shared or Wayland-only by nature - see
docs/TODO.md's own audit entry for the full breakdown).

MenuAction/ContextMenu (row set, labels, row_at hit-testing) move from
crates/wayland/src/context_menu.rs into crates/core/src/context_menu.rs --
pure state and geometry with nothing Wayland-specific in it, so X11
needing the same rows is shared data, not duplicated logic. The Wayland
crate's own context_menu.rs is now a one-line re-export so every existing
crate::context_menu::... call site keeps working unchanged.

X11 has no compositor-level input dispatch to intercept every click the
way Wayland's input/pointer.rs does, so the X11 side
(crates/x11/src/platform/context_menu.rs, new) draws the menu into its own
small override-redirect popup window and grabs the pointer for the
duration so a click anywhere dismisses it, matching the Wayland backend's
own convention. events.rs's ButtonPress handler now reads the real button
number instead of hardcoding every press as a left click - a real latent
bug (right-clicking a titlebar button would have silently performed its
left-click action).

Live-verified end to end in an isolated Xvfb + srdwm --x11 instance: full
row set including the workspace picker, Minimize runs and closes the
menu, a second window's menu dismisses cleanly on outside click, normal
focus/click behaviour continues working afterward.

See docs/TODO.md for the full investigation and verification narrative.

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