fix(lemonade): retry interrupted backend setup

[volen-silo]

Summary

• Retry Lemonade's llama.cpp backend installation once when the backend subprocess fails during first serve.
• Keep the retry at the backend boundary so deterministic archive, disk, and backend-selection failures are not repeated and a healthy embeddable is not re-extracted.
• After the retry fails, recommend rocm engines install lemonade --reinstall before retrying rocm serve.
• Add deterministic unit coverage and a black-box Cucumber scenario for the visible retry count and recovery guidance.

Why: a transient connection loss during first-time Lemonade backend preparation previously aborted rocm serve with a generic error, forcing users to discover the manual recovery sequence themselves.

Risk: low. The change is limited to the backend-install phase, allows one additional attempt, and exposes its failure fixture only in E2E-enabled builds.

Test plan

• cargo xtask e2e -- -n 'Repeated Lemonade preparation failure'
• cargo test -p rocm-engine-lemonade backend_install_
• cargo test -p e2e-cucumber --lib
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets -- --skip therock::tests::extracting_the_sdk_archive_removes_it
• cargo fmt --all -- --check

The skipped archive test is a pre-existing parallel-suite flake on this host (tar temporarily missing from PATH) and passes in isolation.