accessibility text scale: global font multiplier synced to the desktop's text-scaling-factor

New set_text_scale / text_scale process global (clamped [0.5, 3.0]) multiplied into every resolved font size — Canvas::resolve_font for explicit Lengths and the Physical branch of font_px (the Fluid branch routes through resolve_font) — so the whole tree's text follows the accessibility "large text" factor while geometry stays untouched, mirroring GNOME's text-scaling-factor semantics. Unit test covers fonts-scale-geometry-doesn't.
The run loop keeps the factor synced on its own: a watcher thread (event_loop/text_scale.rs) reads org.gnome.desktop.interface text-scaling-factor via gsettings get at startup and streams external changes from gsettings monitor into a calloop channel; on a change the loop stores the factor, invalidates the view caches and repaints main and overlays. Since fonts resolve at paint time nothing else needs rebuilding. Every ltk app tracks the settings slider live with zero app-side wiring, the same way GTK apps follow the key; missing gsettings degrades silently to a fixed 1.0. Embedders driving core::UiSurface (forge) call set_text_scale themselves — the multiplication only runs at widget font resolution, so raw Canvas::draw_text callers keep hand-computed sizes.
architecture.md documents the multiplier in the font-space paragraph and CHANGELOG gains the entry.

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 
 - **`Viewport::local_viewport()`** — resolve the child's viewport-relative (`vw` / `vh` / `vmin`) and fluid `Length`s against the viewport's own rect instead of the root layout viewport the sub-canvas inherits. For fixed-size floating mini-UIs (a phone-shaped panel pinned to a corner of a desktop-wide surface) whose content is calibrated against the panel rect; scroll-like clips should keep the default inheritance.
 - **`ListItem::height( impl Into<Length> )` / `ListItem::font_size( impl Into<Length> )`** — override the theme row height (floored at the label's rendered height so text never clips) and the primary-label font size, mirroring the `Toggle` / `Radio` `height()` builders, so dense menus can trade the touch-target generosity for row density.
+- **Accessibility text scale** — `set_text_scale` / `text_scale` global multiplier (clamped `[0.5, 3.0]`) applied to every resolved font size (`Canvas::resolve_font` and the stock-widget `font_px` path); geometry is untouched. The run loop reads `org.gnome.desktop.interface text-scaling-factor` at startup and follows external changes via a `gsettings monitor` watcher thread, repainting on change — every ltk app tracks the desktop's "large text" setting live with no app-side wiring (silently fixed at 1.0 when `gsettings` is missing). Embedders driving `core::UiSurface` call `set_text_scale` themselves.
 
 - **Per-canvas pixel density** — `Canvas::set_density` pins a canvas (and the sub-canvases derived from it) to its own density factor for `Length::dp` resolution, overriding the process `set_density` global; `Canvas::density` reads the effective value. New `Canvas::resolve_geom` / `Canvas::resolve_font` resolve an explicit `Length` in geometry / font space with the canvas' density — widgets now route caller-supplied lengths through them, so a `dp` override follows the canvas it draws on. The hook for surfaces on outputs whose DPI differs from the process-wide one (an overlay on a second monitor, an embedder with several `UiSurface`s).
 - **`Length::resolve_with_density`** — `Length::resolve` with an explicit density for `LengthBase::Dp`, instead of the process `density()`.