Session management: xdg-session-management-v1 client, mandatory App::app_id / save_state / restore_state, runtime-managed state persistence and clean exit on signals (0.3.0)

Applications built on ltk had no way to come back where the user left them: the toolkit hardcoded `app_id = "ltk"` on every toplevel, never wrote anything to disk, and died on SIGTERM without a chance to save. This release gives the runtime the whole plumbing and asks each application only for the bytes worth keeping, in the spirit of Android's saved-instance state.
The `App` trait gains three mandatory methods, deliberately without default bodies so every application states its position: `app_id()` (reverse-DNS, used for `xdg_toplevel.set_app_id`, the AccessKit application name and the state directory — the `app_id` element of the deprecated `window_config` tuple is now ignored and a one-time warning reports a mismatch), `save_state() -> Option<Vec<u8>>` and `restore_state(Vec<u8>)`. The bytes are opaque; the trait carries no serde bound. Their rustdoc is the contract: when the runtime saves, where the files live, when the bytes come back and when they do not, what must never go in them, and a worked serde_json example.
The runtime persists under `$XDG_STATE_HOME/<app_id>/` (falling back to `~/.local/state`): `session.json` holds the compositor session id, a clean-exit marker and the writer's pid; `state.bin` holds the application bytes. Writes are atomic (temp file + rename, mode 0600, directory 0700) and best-effort. State is saved every 30 s when the bytes changed, once after the event loop exits (which covers `on_close_requested`, `requested_exit` and lost connections), and on SIGTERM/SIGINT — a calloop signal source, installed before any thread exists, now turns those into a clean exit of the loop instead of process death. `restore_state` runs synchronously in `try_run` before the window is created and before the first `view()`, and only when the process is relaunched as part of a session restore (`LTK_SESSION_RESTORE=1`, removed from the environment before the app can spawn children) or when the previous run left `clean_exit: false`; a plain launch starts fresh. A second concurrent instance detects the live pid and runs with persistence disabled rather than clobbering the first.
The compositor side of geometry restore goes through `xdg-session-management-v1`. Neither wayland-protocols nor sctk ship generated code for it yet, so the XML is vendored under `protocols/` and `wayland-scanner` generates the client module in-tree (`src/protocol/`), resolving the crate names through sctk's reexports so the bindings stay on the crate instances sctk links. Before the first commit of a `ShellMode::Window` toplevel the runtime binds `xdg_session_manager_v1`, calls `get_session(reason, stored_id)` and `restore_toplevel(toplevel, "main")`; the three window-creation paths in `run.rs` are folded into one `make_window` helper so the attach always sits immediately before `commit()`. `created` persists the id, `replaced` destroys the objects and stops persisting. Compositors without the global lose only the geometry half. Layer-shell and session-lock surfaces skip the whole machinery.
Every `App` implementor in the tree is updated: the twelve examples (`showcase`, `scroll` and `mini_shell` persist real state; the rest return `None`), both integration tests (`event_loop_flow` gains `save_restore_round_trip`), the in-source and markdown doctests, README, onboarding, cookbook (new recipe "Surviving relaunch: session state") and architecture docs, and the changelog. `src/session_state.rs` carries unit tests over a temporary state directory. `Makefile install` now copies `protocols/` into the cargo registry — without it downstream builds would fail inside the proc-macro — and `debian/copyright` covers the vendored XML.
The trait change is breaking, hence 0.3.0. Also fixes the pre-existing `viewport_tests` module in `render/mod.rs`, which used `Length` without importing it and broke `cargo test`.

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 ### Added
 
+- **`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`.
+- **Clean exit on `SIGTERM` / `SIGINT`** — the runtime installs a calloop signal source and leaves the event loop instead of dying, so `save_state` runs and `ltk::run` returns.
+
 - **`Slider::on_release` / `VSlider::on_release`** — fired once with the final value when the drag ends, so an app can keep an expensive commit (a subprocess, a D-Bus round trip, a compositor reconfigure) off the per-motion `on_change` path and still move the thumb live. The gesture machine emits it from the slider branch of `on_release`; `on_change` alone behaves exactly as before.
 - **`Viewport::local_viewport()`** — resolve the child's viewport-relative (`vw` / `vh` / `vmin`) and fluid `Length`s against the viewport's own rect instead of the root layout viewport the sub-canvas inherits. For fixed-size floating mini-UIs (a phone-shaped panel pinned to a corner of a desktop-wide surface) whose content is calibrated against the panel rect; scroll-like clips should keep the default inheritance.
 - **`ListItem::height( impl Into<Length> )` / `ListItem::font_size( impl Into<Length> )`** — override the theme row height (floored at the label's rendered height so text never clips) and the primary-label font size, mirroring the `Toggle` / `Radio` `height()` builders, so dense menus can trade the touch-target generosity for row density.
@@ -32,6 +36,7 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 ### Changed
 
+- **Breaking: `App::app_id`, `App::save_state` and `App::restore_state` are new mandatory trait methods** — every implementor must add them (shell components: constant id, `None`, no-op). `xdg_toplevel.set_app_id` and the AccessKit application name now come from `app_id()` (previously `"ltk"` / `"ltk-app"`); the `app_id` element of the deprecated `window_config` tuple is ignored. Crate version bumped to 0.3.0.
 - **`Button::icon_size` takes `impl Into<Length>`** and resolves through `Canvas::resolve_geom`, joining `font_size` / `height` / `width`. A bare `f32` still means `Length::px` — every existing call keeps its exact size — but a caller can now pass `Length::widget( n )` to have an icon button follow the widget-scaling mode the way stock icons do. Without it an explicit `icon_size` was the one geometry setter that ignored the mode, so a 21 px back arrow sat next to a `list_item` chevron that fluid sizing had grown well past 21 and looked visibly smaller.
 - **`Length::dp` now applies the density at resolution time, not at construction.** The value carries its design pixels in a new `LengthBase::Dp` variant and `resolve` multiplies by the density in effect when it runs, so a `set_density` change takes effect on the next paint without rebuilding the view's lengths — previously a `dp` value was frozen to the density read when it was constructed. Behaviour is unchanged for code that sets density once at startup.
 - **The GLES image texture cache is now bounded** to 32 MiB of estimated GPU memory with least-recently-drawn eviction (the in-use entry is never evicted). Previously it grew without limit for the canvas' lifetime, so a stream of distinct buffers (photo carousel, video thumbnails) could exhaust GPU memory.