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.

docs/widgets.md

@@ -18,9 +18,9 @@ patterns built from these widgets, see [`docs/cookbook.md`](./cookbook.md).
 - [Continuous controls](#continuous-controls)
   - [`slider`](#slider) · [`vslider`](#vslider) · [`progress_bar`](#progress_bar)
 - [Text input and display](#text-input-and-display)
-  - [`text`](#text) · [`text_edit`](#text_edit)
+  - [`text`](#text) · [`text_edit`](#text_edit) · [`rich_text`](#rich_text)
 - [Decoration and chrome](#decoration-and-chrome)
-  - [`container`](#container) · [`separator`](#separator) · [`img_widget`](#img_widget)
+  - [`container`](#container) · [`separator`](#separator) · [`img_widget`](#img_widget) · [`external`](#external)
 - [Clipping wrappers](#clipping-wrappers)
   - [`scroll`](#scroll) · [`viewport`](#viewport) · [`flex`](#flex) · [`carousel`](#carousel)
 - [Overlays and feedback](#overlays-and-feedback)
@@ -355,6 +355,35 @@ runtime — only what the user sees on screen changes.
 **See also**: the password recipe in
 [`docs/cookbook.md`](./cookbook.md#password-field-with-pam-submit).
 
+### `rich_text`
+
+A wrapped paragraph that carries a `Msg` per clickable link range — the
+ltk counterpart of an Android `Spanned` with `URLSpan` / `ClickableSpan`.
+Unlike `text`, the layout pass emits one hit rect per link *line*, so a
+link that wraps across lines is hit-tested on each of its lines and taps
+land on the link rather than on the whole paragraph.
+
+**When**: body copy with inline links — a terms-and-conditions blurb, a
+chat message with a URL, an "about" screen crediting a project.
+
+```rust,no_run
+# use ltk::{ rich_text, Element };
+# #[ derive( Clone ) ] enum Msg { OpenTerms, OpenPrivacy }
+# fn _ex() -> Element<Msg> {
+let body = "By continuing you accept the Terms and the Privacy Policy.";
+rich_text( body )
+	.size( 14.0 )
+	.link( 30, 35, Msg::OpenTerms )      // byte range of "Terms"
+	.link( 44, 58, Msg::OpenPrivacy )    // byte range of "Privacy Policy"
+	.into()
+# }
+```
+
+Link ranges are **byte** offsets into the content `[start, end)`, drawn
+underlined in `link_color`. `color` sets the non-link text colour, `size`
+the font size (any `Length`), and `font( family, weight, style )` the
+typeface resolved through the active theme on draw.
+
 ---
 
 ## Decoration and chrome
@@ -436,6 +465,39 @@ explicit display dimensions.
 **See also**: [`Image::from_path`](../src/widget/image.rs) helper for
 disk-loaded files (PNG, JPEG via the `image` crate).
 
+### `external`
+
+An escape hatch that reserves layout space and defers its pixels to a
+caller-provided producer, composited in-line with the rest of the tree.
+Two sources:
+
+- `External::cpu( w, h, |canvas, rect| … )` — an immediate-mode CPU
+  drawing closure invoked once per frame with the `Canvas` and the
+  widget's laid-out `rect` (physical pixels). Works on **both** backends
+  and is the way to host a custom `onDraw`-style routine — paths, clips,
+  text — straight onto the canvas with no GL round-trip.
+- `External::new( w, h, ExternalSource::Texture( … ) )` — samples a
+  caller-owned GL texture each frame (a web engine, a video decoder).
+  **GLES only**; the producer keeps the texture and ltk only composites.
+
+**When**: a `VectorDrawable` / Lottie frame, a custom-painted gauge, or
+embedding another renderer's output.
+
+```rust,no_run
+# use ltk::{ External, Element };
+# #[ derive( Clone ) ] enum Msg {}
+# fn _ex() -> Element<Msg> {
+External::cpu( 120.0, 120.0, |canvas, rect|
+{
+	canvas.fill_rect( rect, ltk::Color::rgb( 0.1, 0.1, 0.12 ), 8.0 );
+	// any Canvas primitive: fill_path, set_clip_path, draw_text, …
+} ).into()
+# }
+```
+
+**See also**: the CPU-drawing and path-clip recipe in
+[`docs/cookbook.md`](./cookbook.md#custom-cpu-drawing-and-path-clipping).
+
 ---
 
 ## Clipping wrappers