viewport: local_viewport() opt-out of root-viewport inheritance; list_item: height/font_size builders

`Viewport::local_viewport()` resolves the child's viewport-relative (vw / vh / vmin) and fluid Lengths against the viewport's own rect instead of the root layout viewport the sub-canvas inherits since the fluid-resolution inheritance change. That inheritance is right for scroll-like clips (content renders the same size inside and outside), but it is exactly wrong for a fixed-size floating mini-UI — crustace's phone-shaped quick-settings pill pinned to a corner of a desktop-wide surface resolved its vw text and fluid stock geometry against the whole monitor, inflating the content past the pill's fixed clip and cutting off the bottom stripe. The flag pins the sub-canvas's layout viewport to its own size via the new crate-internal Canvas::set_local_layout_viewport, nested sub-canvases keep propagating the pinned value, and a unit test covers the override plus propagation.
`ListItem::height( impl Into<Length> )` and `ListItem::font_size( impl Into<Length> )` mirror the Toggle / Radio height() builders: override the theme row height (floored at the label's rendered height so the text never clips) and the primary-label font size (subtitle and trailing keep their theme sizes), letting dense context menus trade the stock touch-target generosity for row density.
Docs: widgets.md gains both builder sets, architecture.md documents the layout-viewport inheritance model next to the per-canvas density it parallels, the cookbook slide-in panel recipe notes when the pill needs local_viewport(), and the toggle / radio height() rustdoc demotes its private theme::HEIGHT link to a code span so cargo doc is warning-free again. CHANGELOG entries added.

docs/architecture.md

@@ -197,7 +197,7 @@ Every size in a widget tree is a `Length`, resolved to concrete pixels at layout
 
 Stock widgets do not hard-code either strategy. Each carries a design pixel per dimension (e.g. `button` height 48, font 16) and resolves it through the process-wide `WidgetScaling` mode: `Length::widget(n)` returns `fluid(n)` under `WidgetScaling::Fluid` (the default) or `dp(n)` under `WidgetScaling::Physical`. `set_widget_scaling(mode)` flips it once for the whole app. An explicit `Length` on an individual widget (`button.height(...)`, `text_edit.height(...)`, `font_size(...)`) bypasses the mode entirely — the mode only decides the meaning of the *default* design pixels, never an override the app wrote on purpose.
 
-Both `density()` and `widget_scaling()` are process globals read during layout; set them at startup (or, for density, whenever the surface moves to an output with a different DPI). Because they are global, ltk's own test suite serialises the tests that touch them. Density is also overridable **per canvas**: `Canvas::set_density` pins a canvas (and every sub-canvas derived from it) to its own factor, and all canvas-routed resolution — `geom_px` / `font_px` for stock-widget design pixels, `Canvas::resolve_geom` / `resolve_font` for explicit `Length` values — uses the local density when one is pinned and the process global otherwise. This is the hook for a surface sitting on an output whose DPI differs from the one the global was derived from (an overlay on a second monitor, an embedder with several `UiSurface`s).
+Both `density()` and `widget_scaling()` are process globals read during layout; set them at startup (or, for density, whenever the surface moves to an output with a different DPI). Because they are global, ltk's own test suite serialises the tests that touch them. Density is also overridable **per canvas**: `Canvas::set_density` pins a canvas (and every sub-canvas derived from it) to its own factor, and all canvas-routed resolution — `geom_px` / `font_px` for stock-widget design pixels, `Canvas::resolve_geom` / `resolve_font` for explicit `Length` values — uses the local density when one is pinned and the process global otherwise. This is the hook for a surface sitting on an output whose DPI differs from the one the global was derived from (an overlay on a second monitor, an embedder with several `UiSurface`s). The layout viewport follows the same inheritance model: sub-canvases (scroll and viewport clips, GLES clip layers) inherit the root surface's layout viewport so fluid and viewport-relative values resolve identically inside and outside a clip, and `Viewport::local_viewport()` opts a clip out of it — its child then resolves against the clip's own rect, for fixed-size floating mini-UIs whose content must not scale with the host surface.
 
 `Length` adapts *sizes* to the orientation; to adapt the *structure* of a layout (a row of panels in landscape, the same panels stacked in portrait), branch the view on `ltk::orientation()`. The runtime records the main surface's physical dimensions on every configure (also readable as `ltk::viewport_size()`) and rebuilds the view after each resize, so a `match ltk::orientation() { Landscape => row()…, Portrait => column()… }` follows the window live. The portrait/landscape rule matches `Length::orient` (a square surface counts as portrait). `examples/clip_path.rs` shows the pattern.
 

docs/cookbook.md

@@ -145,6 +145,8 @@ backend renders a hard edge. If your shell must look identical on both
 backends, branch on `ltk::is_software_render()` and skip the fade
 when the software path is active.
 
+If the panel is a fixed-size pill on a much larger surface (a phone-shaped quick-settings card pinned to a corner of a desktop monitor), add `.local_viewport()` to the `viewport` so `vw` / `vmin` and fluid sizes inside resolve against the pill rect instead of the whole surface — without it the panel's content scales with the monitor and overflows the pill.
+
 **See also**: [`Viewport`](./widgets.md#viewport),
 [`OverlaySpec`](../src/app.rs).
 

docs/widgets.md

@@ -175,6 +175,8 @@ When both are set the text sits to the left of the icon.
 its content (theme default 16 px) — lower it when the enclosing view's
 own padding already provides the margin.
 
+`height( len )` overrides the theme row height (56 design px, 68 with a subtitle) and `font_size( len )` the primary-label size (16 design px; subtitle and trailing keep their theme sizes), both `impl Into<Length>`. Use them together where a dense list — a context menu, a compact picker — should trade the touch-target generosity of a settings list for row density; the resolved height is floored at the label's rendered height so the text never clips however aggressive the override.
+
 **See also**: [`pressable`](#pressable) for free-form tappable rows,
 [`scroll`](#scroll) to wrap a list of items in a scrollable container.
 
@@ -578,6 +580,8 @@ viewport( panel_view )
 # }
 ```
 
+By default the clip's sub-canvas inherits the root layout viewport, so viewport-relative (`vw` / `vh` / `vmin`) and fluid `Length`s inside resolve exactly as they would outside the clip — the right behaviour for scroll-like clipping. `local_viewport()` opts out: the child resolves those `Length`s against the viewport's own rect instead. Use it for a fixed-size floating mini-UI — a phone-shaped panel pinned to a corner of a desktop-wide surface — whose content is calibrated against the panel rect and must not scale with the surface hosting it.
+
 **See also**: the slide-in panel recipe in
 [`docs/cookbook.md`](./cookbook.md#slide-in-panel).