Serialize two-device CFU updates

[asasine]

Summary

• await two CFU splitter targets sequentially so shared-bus device update state machines cannot interleave
• still attempt target 1 when target 0 returns None, and retain concurrent join3/join4 behavior
• add focused poll-order tests and host-test feature wiring

Testing

• cargo test --locked -p cfu-service
• cargo fmt --check
• cargo clippy --locked --tests
• cargo hack -p cfu-service --feature-powerset --mutually-exclusive-features=log,defmt,defmt-timestamp-uptime clippy --locked --target x86_64-unknown-linux-gnu
• cargo hack -p cfu-service --feature-powerset --mutually-exclusive-features=log,defmt,defmt-timestamp-uptime clippy --locked --target thumbv8m.main-none-eabihf
• cargo check --locked -p cfu-service -F log --target x86_64-unknown-linux-gnu
• cargo check --locked -p cfu-service -F defmt --target thumbv8m.main-none-eabihf
• cargo machete

[Copilot]

🟡 Changes recommended

The new helper still silently truncates work if output is shorter than input, and the added concurrency tests are brittle by asserting a specific join3/join4 poll order rather than the required concurrency property.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR adjusts the CFU splitter’s internal fan-out helper to serialize the two-device case (preventing interleaving on shared-bus update state machines) while keeping the 3/4-device cases concurrent, and adds targeted tests plus dev-dependency wiring to support host testing.

Changes:

• Serialize map_slice_join when mapping exactly two items (removing join usage), while retaining join3/join4 concurrency for 3–4 items.
• Add unit tests that validate sequential behavior for 2 items and concurrency characteristics for 3–4 items.
• Add std-enabled dev-dependencies for host test builds.

File summaries

File	Description
cfu-service/src/splitter.rs	Serializes the 2-item async mapping path; adds poll-order tests for sequential vs concurrent behavior.
cfu-service/Cargo.toml	Adds dev-dependencies with std features to enable host testing.
Cargo.lock	Records the new critical-section dependency for cfu-service.

Review details

Suppressed comments (1)

cfu-service/src/splitter.rs:208

• map_slice_join silently truncates work if output is shorter than input because zip(input, output) stops at the shorter slice. That can lead to returning true while leaving some input items unprocessed. Add an explicit length check so the function fails deterministically when the slices don’t match.

async fn map_slice_join<'i, 'o, I, O, F: Future<Output = Option<O>>>(
    input: &'i [I],
    output: &'o mut [O],
    f: impl Fn(&'i I) -> F,
) -> bool {
    let mut iter = zip(input.iter(), output.iter_mut());
    loop {

• Files reviewed: 2/3 changed files
• Comments generated: 1
• Review effort level: Lite

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