Embedder primitives: placed-child clipping, offscreen RGBA readback, standalone text measurement

Three additions an embedder needs to drive ltk as the render backend for a retained, externally-owned widget tree, each kept general rather than tied to one consumer.
Stack placed-child clipping. `Stack::push_placed_clipped(e, rect, clip)` places a child at an exact rect like `push_placed` but clips its subtree's drawing to `clip` (in the Stack's coordinate space) — Android's clipChildren: content that overflows, such as a scrolled list row reaching above the list or an inner card past a rounded bubble, is not painted. The Stack child tuple grows an 8th `Option<Rect>` for the clip, and `layout_and_draw` brackets a clipped child with `set_clip_rects`/restore around the recursion. The clip is shifted by the Stack's own origin to match the placed rect (which `Stack::layout` already offsets), so a Stack laid out at a non-zero origin clips in the right place rather than off by that origin.
Offscreen RGBA readback. `Canvas::read_rgba_pixels(out)` reads any canvas into tightly packed straight-alpha RGBA8, top-left row first. Unlike `read_gles_rgba_pixels` it also serves the software backend, un-premultiplying its pixmap, so an offscreen software canvas (an embedder's scratch bitmap) can be read back into a straight-alpha buffer. `Canvas::is_software()` lets a caller branch on the backend — e.g. to honour a real path clip on software but only a bounding rect on GLES.
Standalone text measurement. `measure_text(text, size)`, re-exported at the crate root, measures one line with the default UI font and the system fallback chain, returning `(width, line_height)` in pixels without a live `Canvas` — for an embedder's measure pass that must produce the same metrics the renderer will later use. It is backed by `system_fonts::primary_handle()`, a process-wide cached handle for the primary UI font (the same default a canvas loads, with the bundled-font fallback), which widens `render::helpers::load_default_font_bytes` to `pub(crate)`.

src/render/mod.rs

@@ -262,6 +262,45 @@ impl Canvas
 		}
 	}
 
+	/// Whether this is the CPU (software) backend. Callers that can honour a real
+	/// path clip on the software backend but only a bounding rect on GLES branch
+	/// on this.
+	pub fn is_software( &self ) -> bool
+	{
+		matches!( self, Canvas::Software( _ ) )
+	}
+
+	/// Read this canvas into tightly packed straight-alpha RGBA8, top-left row
+	/// first (`out.len()` must be at least width*height*4). Unlike
+	/// [`Self::read_gles_rgba_pixels`] this also serves the software backend, by
+	/// un-premultiplying its pixmap — used to read back an offscreen software
+	/// canvas (e.g. an Android `Canvas(Bitmap)`) into a straight-alpha buffer.
+	pub fn read_rgba_pixels( &self, out: &mut [u8] ) -> Result<(), String>
+	{
+		match self
+		{
+			Canvas::Software( c ) =>
+			{
+				let pixels = c.pixmap.pixels();
+				if out.len() < pixels.len() * 4
+				{
+					return Err( "read_rgba_pixels: output buffer too small".to_string() );
+				}
+				for ( i, p ) in pixels.iter().enumerate()
+				{
+					let d = p.demultiply();
+					let o = i * 4;
+					out[ o ]     = d.red();
+					out[ o + 1 ] = d.green();
+					out[ o + 2 ] = d.blue();
+					out[ o + 3 ] = d.alpha();
+				}
+				Ok( () )
+			}
+			Canvas::Gles( c ) => c.read_rgba_pixels( out ),
+		}
+	}
+
 	/// Composite an externally-owned GL texture into `dest`. No-op on
 	/// the software backend (no GL state to sample from). Used by
 	/// widgets that host content rendered by an external producer —