ltk: ext-session-lock-v1 client surface mode, plus a read-only mode for text fields

Add a third Wayland surface type to the runtime so an ltk `App` can be a screen locker, alongside the existing xdg-shell window and wlr-layer-shell surfaces. A new `ShellMode::SessionLock` makes `run()` bind `ext_session_lock_manager_v1` and request the lock at startup; the lock surface itself is created in the new `SessionLockHandler::locked` callback (one surface on the first advertised output) and replaces the `SurfaceKind::PendingLock` placeholder the main surface holds until the compositor grants the lock. The `configure` event routes through the same `on_configure` path as layer and xdg surfaces, so sizing and rendering are unchanged, and `finished` (the compositor denied or ended the lock) tears the loop down. The whole thing is additive and opt-in: the `Window` and `Layer` paths are untouched and nothing enters lock mode unless an `App` returns `ShellMode::SessionLock`, so existing apps are unaffected — the only non-additive edits are the two exhaustive `match`es on `SurfaceKind` (`wl_surface` / `try_wl_surface`), which gain arms for the two new variants.
Doing the locker as a first-class surface rather than compositing a static texture into an offscreen `UiSurface` is the whole point: the compositor gives the lock surface keyboard focus, so ltk's existing text-input, editing, focus and IME machinery works inside the lock exactly as on any other surface — cursor, click-to-focus, Tab, character input. A locker built on top of this is just a normal interactive ltk app that happens to be presented on the lock layer, with no special input plumbing on the compositor or the app side.
`App::requested_exit()` is the new way an app asks the runtime to tear the surface down and leave the loop; it is polled after every batch of `update`s. It exists because of the one hard invariant of `ext-session-lock-v1`: a locker that disconnects without sending `unlock` leaves the compositor's outputs blanked forever — that is the protocol's deliberate anti-bypass guarantee. So when `requested_exit()` returns true and the surface is a session lock, the loop calls `session_lock.unlock()` and round-trips the connection before setting `exit_requested`, lifting the lock cleanly; for a `Window` or `Layer` surface there is no lock and it simply exits. The consequence for lock apps is that they must stop calling `process::exit` from the lock path and instead flip a flag they return from `requested_exit()`.
`text_edit` gains a `read_only( bool )` builder. A read-only field still renders its box and value in the normal field style but takes no keyboard focus and accepts no input: `Element::is_focusable` and `Element::is_text_input` now return false for a read-only `TextEdit`, which keeps it out of the Tab cycle, off the keyboard-edit path, and stops the cursor from ever being drawn on it. The flag is carried through `map_msg` so it survives `Element::map`. This is for presenting a known, non-editable value in the same visual idiom as the editable fields beside it — for example the already-known user shown on a session lock, where letting that field take focus or blink a cursor would be wrong.
The `shell_mode()` doc comment and the README now list the `SessionLock` surface type and point at `requested_exit()` for the unlock. Two warnings are cleared along the way: the runtime no longer stores the `SessionLockState` after requesting the lock — it has no `Drop`, so the manager object outlives the dropped handle inside the connection and the lock lifecycle runs entirely off the returned `SessionLock`, which removes a never-read field — and a pre-existing rustdoc `private_intra_doc_links` warning in `list_item` (a public doc comment linking to the private `theme::ICON_SIZE`) is downgraded to plain code formatting.

src/widget/text_edit/mod.rs

@@ -153,6 +153,9 @@ pub struct TextEdit<Msg: Clone>
 	/// has `secure( true )` and the explicit flag becomes redundant
 	/// — the toggle controls the visibility from then on.
 	pub password_toggle: Option<( bool, Msg )>,
+	/// When `true`, the field renders its box and value but takes no keyboard
+	/// focus and accepts no input — a read-only display styled as a text field.
+	pub read_only:   bool,
 }
 
 impl<Msg: Clone> TextEdit<Msg>
@@ -181,6 +184,7 @@ impl<Msg: Clone> TextEdit<Msg>
 			font_size:       theme::FONT_SIZE,
 			select_on_focus: false,
 			password_toggle: None,
+			read_only:       false,
 		}
 	}
 
@@ -356,6 +360,14 @@ impl<Msg: Clone> TextEdit<Msg>
 		self
 	}
 
+	/// Render the field read-only: shows the value styled as a field but
+	/// takes no focus and accepts no input.
+	pub fn read_only( mut self, on: bool ) -> Self
+	{
+		self.read_only = on;
+		self
+	}
+
 	/// Assign a stable identifier for focus management.
 	pub fn id( mut self, id: WidgetId ) -> Self
 	{
@@ -487,6 +499,7 @@ impl<Msg: Clone> TextEdit<Msg>
 			font_size:       self.font_size,
 			select_on_focus: self.select_on_focus,
 			password_toggle: self.password_toggle.clone().map( |( v, m )| ( v, ( *f )( m ) ) ),
+			read_only:       self.read_only,
 		}
 	}
 }