Bundled theme icon data

The coherent Rust shell now receives Yaru, Yaru-purple and Yaru-blue in XDG_DATA_DIRS, plus their Humanity and hicolor inheritance. Catppuccin selects Yaru-purple and Catppuccin Latte selects Yaru-blue. Other user-selected icon themes still require installation; existing app-specific icons remain fallbacks.

The icon data comes from the flake-pinned nixpkgs yaru-theme 25.10.3 package, upstream https://github.com/ubuntu/yaru/tree/25.10.3. Its declared licenses are CC-BY-SA-4.0, GPL-3.0-or-later, LGPL-2.1-only and LGPL-3.0-only; these are upstream art, not original project art. The architecture-independent icon trees are copied from the native package; GTK themes, sounds and executables are excluded. All selected Yaru trees and their relative symlinks are preserved unchanged. Humanity and hicolor retain their pinned Nix references.

Host package proof on 2026-09-24, branch impl/theme-icons, base c0ffd54c:

nix build --no-link --print-out-paths .#handheld-theme-icons

PASS output /nix/store/in3nc21zx02xiz2rdp5bpq36719x3axd-handheld-theme-icons-25.10.3; derivation /nix/store/rbhinhb3943pk3y47jxbjvgmzqj373ni-handheld-theme-icons-25.10.3.drv. All five index files are readable; the three copied Yaru trees have no broken symlinks. Upstream Humanity has 20 dangling aliases, preserved rather than rewritten. The existing resolver ignores missing targets and retains fallback. Runtime references contain only Humanity and hicolor data packages, not the native Yaru GTK package. This is package proof; actual Rust decoder use and physical icon/theme switching are separate evidence gates.