Rust theme chooser host checkpoint

Source base: 8b8b4666ed78503f807e8c7a10ce559ad717209b (the pinned k230-theme bridge is already present). Frontend source: 4efe3409, 66ce699d, corrected f80eedcd19e563d2804fe682ed9d75deac9a3adb. This is a host and cross-build checkpoint, not an installed or physical touch result.

The persistent Rust shell now exposes Themes within Settings. The chooser requests a bounded asynchronous list, previews a selected theme without activating it, displays its reported palette and background choices, and sends an explicit Apply request with the preview’s exact generation and selected background ID. A video background remains visible as unavailable because the Rust shell has no video renderer. Cancel/back leaves the compositor-owned live deck and theme unchanged; an activation already dispatched cannot be cancelled locally. Worker or backend errors remain visible in the chooser.

Host checks in nix/rust-shell-client:

cargo fmt --all -- --check
cargo test --offline
cargo clippy --offline --lib --bin k230-shell-rust -- -D warnings
K230_THEME_FIXTURE_PNG=/tmp/k230-rust-theme-preview.png cargo test --offline theme_list_preview_and_scroll_paint_distinct_handheld_scenes

Result on 2026-09-24 at corrected source f80eedcd: 32 library, 6 main, 10 appearance, 5 background, and 8 service integration tests passed; strict library/binary Clippy passed. The added regression rejects a stale value-equal List reply after reopen and blocks Apply after failed background selection until another preview succeeds. The 568×1232 fixed-content preview PNG was opened and visually checked for readable heading, palette, background list, unsupported-video label, and explicit Apply. That /tmp capture is a host fixture, not device evidence and not a private application capture.

Exact narrow cross-build from committed f80eedcd passed:

nix build .#handheld-shell-rust --max-jobs 1 --cores 4 --no-link --print-out-paths

Derivation: /nix/store/w9ywvarl0zdmk7c06wwk1142fcxvfn7m-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0.drv. Output: /nix/store/rlb1kr180g8khgnqx83kfynl5lyvv0h5-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0. The first 66ce699d package also built, but the corrected package is the QEMU input.

Still open: integrated installed image, a paired Sway/Rust QEMU route and backend transaction check, real touch scrolling/preview/Apply, and the sibling theme proposal’s full background thumbnail/motion and physical acceptance gates. No OpenSpec task is checked by this source/build checkpoint alone.