Rust shell reveal stream: host checkpoint

Source: nix/rust-shell-client/src/protocol.rs, src/main.rs, and src/render.rs in this commit. This is an opt-in Rust client checkpoint; it is not an installed session or physical touch result.

The client accepts the compositor’s bounded newline JSON v1 begin, update, finish, and cancel messages on the private 0600 k230-shell-rust.sock. A complete finish followed by EOF retains the settled-open drawer or shade; an incomplete stream settles closed. Sequence and route must remain consistent, malformed or oversized lines abort, and a five-second idle peer timeout bounds abandoned contacts. The compositor continues to own gesture classification and live app or deck pixels. The client renders a transparent full-output overlay with no input region during tracking; the panel receives input only after the open settle. Cairo/Pango scene shaping is cached across progress updates, which copy/translate the prepared scene into bounded SHM buffers. This says nothing yet about presented frame rate on the board.

Host proof from the source tree:

cargo fmt --manifest-path nix/rust-shell-client/Cargo.toml --check
cargo test --manifest-path nix/rust-shell-client/Cargo.toml --locked
cargo clippy --manifest-path nix/rust-shell-client/Cargo.toml --locked --all-targets -- -D warnings
git diff --check

All completed successfully on 2026-09-23. The Rust unit tests include split socket writes, mid-drag reversal, cancellation, premature EOF, completed-finish EOF, input progress, renderer cache reuse, and geometry bounds.

The exact committed source a8fa5d3283e679724a554d9db52203b98f90ce0e was built with:

nix build "git+file://$PWD?rev=a8fa5d3283e679724a554d9db52203b98f90ce0e#handheld-shell-rust" --max-jobs 1 --cores 4 --no-link --print-out-paths

The target derivation was /nix/store/f7kn0zqldgh9bkj48k2pjx2b966wd3k4-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0.drv; output /nix/store/kn3x90rplwfrf83z1nk0jjpa0whigrk1-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0. file reports ELF64 RISC-V LP64D. nix path-info -S reports an 86,450,072-byte recursive closure. This is package proof only. Paired QEMU pixels/input routing and physical board observations remain open.

Review found that buffered progress lines incurred one 16 ms event-loop pause each. Commit 692daa9a0dc788c081af03d27af4691b93de7b5b drains at most 64 records per pass, coalesces contiguous updates of the same gesture while preserving begin/terminal ordering, and reads K230_SETTINGS_REDUCED_MOTION=1 for the shorter settle. Exact source extracted with git archive 692daa9a passed 13 host tests (11 library, 2 route server) and Clippy with -D warnings. Its immutable build passed with:

nix build "git+file://$PWD?rev=692daa9a0dc788c081af03d27af4691b93de7b5b#handheld-shell-rust" --max-jobs 1 --cores 4 --no-link --print-out-paths

Derivation /nix/store/kaa33n1b2l1vlyjzsd42si2x229qar0v-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0.drv produced /nix/store/h56a3xk2nmjh29vi1yd1g0q5iks4rpg3-k230-shell-rust-riscv64-unknown-linux-gnu-0.1.0. This revised binary has not been physically tested. The paired QEMU result for earlier a8fa5d32 is recorded separately by the compositor owner and must not be attributed to the revised build.