pattern_grid: explicit side through PatternGrid::size

The grid's preferred size was the theme's fluid constant (264 px at the reference surface) capped by the width the parent allocates, so a caller could only shrink it: a lock screen that wants the grid to span most of a phone's width had no way to ask for it, and a container's max_width did nothing past the default. PatternGrid gains a size field with a size( impl Into<Length> ) builder; preferred_size resolves it through Canvas::resolve_geom like the other geometry overrides and still caps it at max_width, and map_msg carries it across. Without the builder the fluid default applies as before.
docs/widgets.md describes the builder next to hide_stroke and error; CHANGELOG lists it under Unreleased.

CHANGELOG.md

@@ -6,6 +6,7 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 ### Added
 
+- **`PatternGrid::size( impl Into<Length> )`** — explicit side for the square grid, resolved through `Canvas::resolve_geom` like the other geometry overrides; without it the theme's fluid default still applies. A lock screen can now size the grid from the surface (a share of the width in portrait, of the free height in landscape) instead of being capped at the default.
 - **`xdg-session-management-v1` (client)** — before the first commit of a `ShellMode::Window` toplevel the runtime binds `xdg_session_manager_v1`, issues `get_session( reason, stored_id )` and `restore_toplevel( toplevel, "main" )`, so a supporting compositor restores window geometry on every launch. Bindings are generated in-tree from the vendored XML under `protocols/` with `wayland-scanner` (`src/protocol/`).
 - **Runtime session persistence** — `$XDG_STATE_HOME/<app_id>/session.json` (compositor session id, clean-exit marker, pid) plus `state.bin` (the bytes from `App::save_state`), written atomically with mode `0600`; saved every 30 s when the bytes changed, on close and on signal; handed back through `App::restore_state` before the first frame only on a session restore (`LTK_SESSION_RESTORE=1`) or after an unclean exit. Module `src/session_state.rs`.
 - **`Layer::Window` overlays** — an `OverlaySpec` with `layer: Layer::Window` is materialised as an ordinary `xdg_toplevel` of the given fixed size instead of a layer-shell surface: decorated, stacked and listed by the compositor like an application window. Anchor, exclusive zone and keyboard exclusivity are ignored; the compositor's close button delivers `on_dismiss`. `App::overlay_title( id )` (defaulted to the app id) names it. Layer-shell apps now bind `xdg_wm_base` when available so they can open one.