ltk: responsive padding/spacing and scrolling, expanded theme palette, and bundled Adwaita cursors

A mixed pass over the default theme and the layout/input core, plus the toolkit's own cursor set. Grouped by area below.
== Responsive sizing ==
Add `Length::dp( px )` — a "design pixel". It interprets `px` against a configurable reference vmin (default 412 px, the eydos mobile reference width) and returns `Vmin( px / reference * 100 ).clamp( px * 0.7, px * 1.5 )`, so a value authored against a mock-up scales with the surface without collapsing on tiny screens or ballooning on a 4K desktop. The reference is process-global, set via `set_design_reference()` and read via `design_reference()` (stored as f32 bits in an AtomicU32); both are re-exported from `lib.rs`.
Make container and grid insets relative. `Container`'s four padding fields become `Length` instead of `f32`; every setter (`padding`, `padding_h`, `padding_v`, `padding_top`/`right`/`bottom`/`left`) now takes `impl Into<Length>`, so existing `f32` call sites keep compiling via the `From<f32>` shim. The values are resolved against the viewport in `Container::preferred_size` and in the container draw path (`draw/layout.rs`). `WrapGrid`'s `spacing_x`, `spacing_y` and `padding` get the same treatment, with a `resolved( canvas )` helper funnelling the per-frame resolution and `grid()` seeding `Length::px` defaults. Container tests now compare against `Length::px( … )`.
== Scrolling ==
`Scroll::preferred_size` is now axis-aware. A horizontal-only scroll reports its child's natural height rather than claiming all remaining vertical space, so it no longer steals Y from its siblings when it sits inside a `Column`; vertical and both-axis scrolls keep the spacer-like `( max_width, 0.0 )`. `Column`'s space-distribution correspondingly treats a `Scroll` as a vertical space-claimer only when its axis allows Y.
Disambiguate nested scroll viewports by direction. On press the gesture state now collects every scroll viewport under the point (`scroll_candidates`, innermost first) instead of committing to one; on the first 8 px of motion it locks onto the candidate whose axis matches the dominant direction (`scroll_locked`), so a horizontal scroller nested inside a vertical list no longer grabs the wrong axis. The pointer scroll hit test is aligned to the same innermost-first ordering.
== Theme palette ==
`themes/default/theme.json` gains named colours (green / green-deep, yellow, orange / orange-deep, pink / pink-soft, sky-deep, error / error-soft, neutral-tertiary) and new semantic slots in both light and dark modes: `danger`, `text-tertiary`, `accept`, `chip` / `chip-active` / `chip-active-fg`, and `avatar-1` … `avatar-9`.
== Cursors ==
Bundle GNOME's Adwaita cursor theme — the cursors GNOME Shell uses — into `themes/default/cursors/` so a Wayland compositor can draw consistent, complete pointers for ltk applications without the toolkit rasterising cursors itself and without depending on adwaita-icon-theme being installed on the target. The cursors are copied verbatim in XCursor binary format: 35 image files, one per CSS/freedesktop cursor name (default, text, pointer, *-resize, …), plus 27 customary X11 alias symlinks (arrow → default, hand2 → pointer, …); a sibling `cursor.theme` makes the tree a valid XCursor theme. The existing `ltk-theme-default.install` copies `themes/default` recursively, so the directory ships with no packaging change. Applications keep declaring a `CursorShape` per widget over `wp_cursor_shape_v1`; the compositor resolves it against the active theme's `cursors/` directory by name, and the set covers all 34 `CursorShape` variants.
Document the set in `themes/default/cursors/README.md` (what it is, the XCursor layout, the full shape list, how the compositor consumes it, guidance for forks) and `themes/default/cursors/LICENSE.md` (attribution and licence options, modelled on the icons catalogue LICENSE). `lib.rs` lists the cursors in its third-party-assets section.
Close out licensing in `debian/copyright`: a `Files: themes/default/cursors/*` paragraph records the upstream dual offer (CC-BY-SA-3.0 or LGPL-3, and CC-BY-SA-4.0 for the newer assets) attributed to the GNOME Project, with standalone CC-BY-SA-3.0, CC-BY-SA-4.0 and LGPL-3 paragraphs (summary-plus-canonical-URL for the CC licences, matching the existing CC-BY-4.0 entry; LGPL-3 referencing /usr/share/common-licenses/LGPL-3). The files are unmodified from upstream, so there is nothing to declare under the ShareAlike "indicate if changes were made" clause.
Add `tests/cursor_assets.rs`: every `CursorShape` name resolves to a valid XCursor file (Xcur magic, following symlinks), `cursor.theme` is present, no entry is a dangling symlink, and the expected-name list stays in sync with the enum's 34 variants.

themes/default/cursors/sb_v_double_arrow

@@ -0,0 +1 @@
+ns-resize