Glyphs on demand: FontFace replaces fontdue, DejaVu ahead of the CJK collection in the fallback chain; App::take_clipboard_request and App::on_pointer_secondary

Every face fontdue opened was parsed whole: `Font::from_bytes` walks the `cmap` and builds the outline of every glyph before returning. For the theme's Sora that is a few hundred glyphs and nobody notices. For a fallback it is the whole file: the first string with one arrow or one bullet resolved through the chain to `NotoSansCJK-Regular.ttc`, and a compositor or a notification daemon that had drawn a dozen labels was suddenly holding 330 MB of outlines for 44 810 glyphs, after 1.7 s of CPU, to paint the hundred or so glyphs a screen ever shows. Measured on forge: 240 MB before the first app-switcher label, 542 MB after it, and never given back, because the table is live, not garbage.
`FontFace`. The new `src/font_face.rs` is the face type the whole crate now rasterises with, built on the `ttf-parser` that rustybuzz already brings in and on tiny-skia, which already fills paths. `from_bytes( data, index )` keeps the bytes and reads only what a face needs up front: units per em and the vertical line metrics. A `ttf_parser::Face` is rebuilt on every use rather than kept, since parsing one is table-offset arithmetic, in the noise next to the outline work it guards. Each glyph's outline is extracted the first time it is asked for, through `Outliner`, into a `tiny_skia::Path`, and cached for the process lifetime in a `HashMap<u16, Arc<Outline>>`; codepoint lookups are cached the same way. Rasterising fills that path into a `Mask` and hands back the 8-bit coverage, top row first, which is what both canvases stored before. The metrics keep fontdue's conventions, so no widget changed: sizes scale by `px / units_per_em`, `Metrics::xmin` / `ymin` are whole-pixel placements with the fractional remainder folded into the raster offset, `bounds` is the outline box in subpixels, `LineMetrics` carries ascent, descent, line gap and their sum. One thing is deliberately not what `glyf` declares: the box comes from the points the outline actually reaches, quadratic and cubic extrema included, because declared boxes may be loose and a box wider than the ink shifts the glyph on the pixel grid. Tests cover the turning-point arithmetic and the line-metric scaling. `FontFace`, `Metrics`, `LineMetrics` and `OutlineBounds` are exported, since `Canvas::font` and `font_for` hand them out.
What changed around it. `fontdue` leaves `Cargo.toml`. The two canvases, their text and setup modules, the registry, the rich text and text widgets swap `Arc<Font>` for `Arc<FontFace>` and nothing else; the glyph caches still key on `(glyph_id, size_bits, font_id)`. `FontRegistry` now reads a source into an `Arc<Vec<u8>>` and gives the face the bytes it will pull outlines from, so the registry no longer owns a second copy; its parse error names the file rather than a library message. `FontHandle` keeps `font` and `bytes` side by side, now naming the same buffer, so shaping needs no second read and the comment that explained the duplication is gone. `system_fonts` builds fallback faces from the bytes each slot already holds, and the slot stays two-staged: the bytes are read once to probe coverage with `ttf-parser`, the face is built only when a glyph in that slot is needed, and a face that is built no longer costs anything until it draws.
The chain. DejaVu Sans moves from last resort to just before the CJK collection. It owns the geometric shapes, arrows and box-drawing blocks, which are what a user interface most often needs outside its own family; behind the collection, every one of those pulled the 19 MB file in, since probing a slot for coverage reads it whole. The CJK entries stay last for the same reason.
The two hooks are for a host that embeds another UI, trust in this case, and complete the ones `App` already had. `take_clipboard_request` is the counterpart of nothing: an app could paste from the system clipboard through its text fields but had no way to put text into it. Return `Some( text )` once and the run loop stores it and publishes it as the seat's selection through the data device, the same path `handle_copy` takes for Ctrl+C in a text field, so other applications see it. `on_pointer_secondary` reports a right-button press that no widget of ltk's own answered, in physical pixels like `on_pointer_button`: `on_pointer_right_click` already dispatched a widget's `on_long_press` and the built-in text menu, and in the remaining case only dismissed an open menu; it now also tells the app, which can forward the press to an embedded view that has a context menu of its own.
Not changed. Variable fonts are read at their default instance, as before. Colour glyphs, `COLR` or bitmap, are not rendered by either rasteriser; the outline path draws what it can. The glyph caches are still unbounded per face, but they now hold what was drawn, not what the file contains.

src/app.rs

@@ -868,6 +868,9 @@ pub trait App: 'static
 	/// Return `true` once to drop the keyboard focus, which folds the on-screen keyboard; a [`take_focus_request`](Self::take_focus_request) in the same cycle wins.
 	fn take_blur_request( &mut self ) -> bool { false }
 
+	/// Return `Some( text )` once to put it in the clipboard, for other applications as well.
+	fn take_clipboard_request( &mut self ) -> Option<String> { None }
+
 	/// Called on every pointer motion (mouse move, button press, button
 	/// release). `x`, `y` are in physical pixels of the main surface,
 	/// the same coordinate space as widget rects. Default: no-op.
@@ -958,6 +961,9 @@ pub trait App: 'static
 	/// classifier collapses press+release into a single event.
 	fn on_pointer_button( &mut self, _x: f32, _y: f32, _pressed: bool ) {}
 
+	/// A secondary-button press that no widget of ltk's own answered, in physical pixels. Default: no-op.
+	fn on_pointer_secondary( &mut self, _x: f32, _y: f32 ) {}
+
 	/// Called on every keyboard event (press *and* release) before the
 	/// regular focus-aware dispatch ([`Self::on_key`] /
 	/// [`Self::on_key_with_modifiers`]) runs. Default: no-op.