render, types, ci: resolution-time dp with per-canvas density, bounded GLES image cache, clippy gate, backend capability matrix

Length::dp no longer collapses to absolute pixels at construction: the design value travels in a new LengthBase::Dp variant and the density multiplication happens when the length is resolved. Previously dp( n ) baked in whatever density() returned at view-build time, so correctness across output changes depended on the view being rebuilt after set_density and in that order; now a density change is picked up by the very next paint with no reconstruction. Length::resolve keeps its signature (process density), and the new Length::resolve_with_density takes an explicit factor. dp becomes const in the bargain.
Density also becomes overridable per canvas, the first step towards surface-local responsive state. SoftwareCanvas and GlesCanvas carry a density: Option<f32> analogous to the layout_viewport introduced for sub-canvas fluid resolution: None means "use the process global", Canvas::set_density pins a local factor, and sub-canvases inherit it. All canvas-routed resolution honours it — geom_px / font_px for stock-widget design pixels, and the new Canvas::resolve_geom / resolve_font for explicit Length values, which every widget now uses in place of the raw l.resolve( canvas.viewport_layout(), EM ) pattern (row, column, wrap_grid, spacer, container, separator, button, text, rich_text, text_edit, list_item, vslider, image, and the container draw path). Overlay sizing keeps resolving against the main surface with the global density, which is what it describes. New tests cover explicit-density resolution, resolution-time application, the local-over-global override and sub-canvas inheritance.
The GLES image texture cache is now bounded. It was content-keyed but unbounded and never evicted, so a stream of distinct buffers — a photo carousel, video thumbnails — grew GPU memory for the lifetime of the canvas. The cache now tracks an estimated byte total (RGBA8, w × h × 4) against a 32 MiB budget and evicts least-recently-drawn textures on insert; the most recent entry is never evicted, so a single texture larger than the whole budget still draws and simply owns the cache until replaced. Drop-time cleanup is unchanged: drain deletes whatever the map holds.
CI gains a Clippy step (workspace, all targets, test-support, -D warnings) sharing the build cache of the test job, with make clippy mirroring the invocation locally and CONTRIBUTING listing it. Run make clippy locally before pushing the first time — the gate has not seen the tree yet and pre-existing lints will fail CI until addressed.
docs/backends.md formalises the software/GLES capability matrix that was previously scattered across per-method rustdoc: parity set (fills, strokes, text, images, paths, path clips), graceful degradations on software (flat-fill gradients, no shadows, no backdrop blur, hard bottom edge), GPU-only features (external textures), the shared Oklab-fallback limitation, and the cross-backend blit panic. Linked from README, onboarding and architecture's known-gaps list, which now states the parity gaps explicitly. The dp/density prose in architecture.md, lib.rs and the Length rustdoc is updated for resolution-time semantics and the per-canvas override.

Makefile

@@ -5,7 +5,7 @@ DOCDIR   ?= /usr/share/doc/libltk-doc/html
 # ignore filesystem entries with the same name — without this `examples`
 # silently no-ops because the `examples/` directory exists, and `doc`
 # would do the same once `target/doc` is around.
-.PHONY: all test doctest-md stylecheck audit doc install examples clean distclean
+.PHONY: all test doctest-md stylecheck clippy audit doc install examples clean distclean
 
 all:
 	cargo build --release
@@ -25,6 +25,11 @@ doctest-md:
 stylecheck:
 	./scripts/style-check.sh
 
+# Semantic lints, mirroring the CI invocation. Warnings are errors so a
+# local run predicts the CI verdict.
+clippy:
+	cargo clippy --workspace --all-targets --features test-support -- -D warnings
+
 audit:
 	@command -v cargo-audit >/dev/null 2>&1 || cargo install cargo-audit --locked
 	cargo audit