Bump to 0.2.0: SW/GLES paint parity, shared font resolution, FrameState refactor, docs and packaging fixes

Rendering parity (software ↔ GLES). The software backend now rounds glyph pen positions and image destinations to the nearest integer pixel, matching what the GLES backend already did; previously it truncated, so text and 1:1 images could land up to half a pixel off between the two backends and the bilinear sample read ~1 px softer than the source. Gradients and shadows are deliberately left unimplemented on the software backend, and the GLES multi-rect `glScissor` clip is left coarse on purpose: making it exact would need stencil bits the EGL config does not carry, or routing the partial-redraw path through the offscreen clip layer, which would break its `fill` / `clear_rects_transparent` scissor semantics. Adds software-backend pixel tests covering the snapping.
Font resolution unification. The system-font candidate chain, `find_font_opt` and `load_default_font_bytes` lived in two copies (`render/helpers` and `gles_render/helpers`) that had already diverged — one resolved through `find_font_opt`, the other inlined the candidate loop — and now live once in `system_fonts`. The two per-backend `OnceLock` default-font caches and `primary_handle` collapse into a single `system_fonts::default_handle`. Module docs and the `font_registry` caller are updated accordingly.
Shared image validation and rect inflation. `draw_image_data`'s dimension check and its one-line warning were byte-duplicated across both backends and are now `render::helpers::validate_rgba_dims`. The six manual symmetric `Rect`-inflate literals in the GLES primitives reuse the existing `Rect::expand`.
FrameState and DrawCtx de-duplication. The eleven `SurfaceState` fields the draw pass owns and threads through `DrawCtx` — `widget_rects`, the cursor / selection maps, the scroll state, `accessible_extras`, `prev_focused` / `prev_hovered` / `prev_pressed` — move into a `FrameState` sub-struct. The per-frame `build_draw_ctx` / `commit_draw_ctx` helpers can then borrow `&mut ss.frame`, disjoint from `ss.canvas` and `ss.pool`, so the four frame paths (software / GLES × full / partial) replace their duplicated `DrawCtx` construction and write-back with a single helper call each. A whole-`SurfaceState` borrow could not express this (partial borrows do not cross function boundaries), which is why the helpers take the sub-struct. `content_dirty` stays on `SurfaceState` — it is an invalidation flag, not frame state — and is reset at the call site.
rich_text tests. Adds the previously-missing `tests.rs` for the `RichText` widget: one hit rect per visual line a link spans (the widget's core invariant), the single-line and no-link cases, preferred-size growth with hard line breaks, and `map_msg` range preservation — all headless against a software `Canvas`, with line counts forced by `\n` so they do not depend on any system font's measured width.
Documentation. Fills the rustdoc gaps on the embedder-facing surface: `core::UiSurface` accessors, the `egl_context` public API, the `GlesCanvas` methods, `theme::typography` and `theme::error`, and the `RichText` / `Text` builders; adds a crate-level "Rendering backends" overview. `CHANGELOG.md` is added (0.2.0 / 0.1.0), and `docs/widgets.md` / `docs/cookbook.md` gain `rich_text`, `external`, and the CPU-draw / path-clip / externally-laid-out-tree recipes. `debian/changelog` gets the 0.2.0-1 entry. Private intra-doc links to `system_fonts` are demoted to code spans so `cargo doc` is warning-free.
Packaging. The `libltk-dev` registry crate shipped a `Cargo.toml` declaring the `lookup` bench while the `Makefile` install copied only `src/`, so Cargo refused to parse the manifest over a missing `benches/lookup.rs`; the install now ships `benches/` as well (the file alone satisfies the parse — criterion is a dev-dependency and is not resolved when the crate is consumed as a library). `Cargo.toml` is bumped to 0.2.0 to match the package version and the `ltk-0.2.0` registry directory.

src/input/pointer/press.rs

@@ -43,9 +43,9 @@ impl<A: App> AppData<A>
 		let pos = self.surface( focus ).to_physical( event.position.0, event.position.1 );
 		self.pointer_pos = pos;
 		self.app.on_pointer_move( pos.x, pos.y );
-		let hit_idx = find_widget_at( &self.surface( focus ).widget_rects, pos );
+		let hit_idx = find_widget_at( &self.surface( focus ).frame.widget_rects, pos );
 		let lp_msg = hit_idx.and_then( |idx|
-			find_handlers( &self.surface( focus ).widget_rects, idx )
+			find_handlers( &self.surface( focus ).frame.widget_rects, idx )
 				.and_then( |h| h.long_press_msg() ) );
 		if let Some( msg ) = lp_msg
 		{
@@ -53,7 +53,7 @@ impl<A: App> AppData<A>
 			self.surface_mut( focus ).request_redraw();
 		} else {
 			let is_text = hit_idx.and_then( |idx|
-				find_handlers( &self.surface( focus ).widget_rects, idx )
+				find_handlers( &self.surface( focus ).frame.widget_rects, idx )
 					.map( |h| h.is_text_input() ) ).unwrap_or( false );
 			if is_text
 			{
@@ -170,7 +170,7 @@ impl<A: App> AppData<A>
 		let outcome =
 		{
 			let ss = self.surface_mut( focus );
-			let result = ss.gesture.on_press( pos, &ss.widget_rects, &ss.scroll_rects );
+			let result = ss.gesture.on_press( pos, &ss.frame.widget_rects, &ss.frame.scroll_rects );
 			// Mark this gesture as mouse-driven so the
 			// gesture machine's 6 px stray-cancel skips
 			// the drag-start / long-press slots — mouse
@@ -199,7 +199,7 @@ impl<A: App> AppData<A>
 		if let Some( idx ) = outcome.hit_idx
 		{
 			let immediate = {
-				let handlers = find_handlers( &self.surface( focus ).widget_rects, idx );
+				let handlers = find_handlers( &self.surface( focus ).frame.widget_rects, idx );
 				if matches!( handlers, Some( WidgetHandlers::Button { repeating: true, .. } ) )
 				{
 					handlers.and_then( |h| h.press_msg() )
@@ -222,7 +222,7 @@ impl<A: App> AppData<A>
 		// under the cursor instead of just positioning.
 		if let Some( idx ) = outcome.hit_idx
 		{
-			let is_text = find_handlers( &self.surface( focus ).widget_rects, idx )
+			let is_text = find_handlers( &self.surface( focus ).frame.widget_rects, idx )
 				.map( |h| h.is_text_input() ).unwrap_or( false );
 			if is_text
 			{