test(workflow): lock down affected unit IDs

[donbeave]

Change

Add executable selection fixtures against the checked-in generated project configuration and real temporary git histories.

• Cargo.lock-only version diff selects exactly docker, rust-velnor-bench, rust-velnor-runner, and rust-velnorctl.
• Single velnor-client source diff selects exactly the target plus documented dependents/prerequisites, with exact full_units.

No production selection logic or generated workflow changed.

Verification

• cargo fmt --all -- --check
• cargo test --locked -p velnor-workflow --all-features: 143 passed
• git diff --check

Advances CI_CD_PERFORMANCE_GOAL.md section 5 P2 and section 8 acceptance item 3.

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[donbeave]

Closing as superseded: the affected-unit-ID coverage this adds (Cargo.lock-only change selects the version-bump allowlist; single-crate change selects the exact dependency closure) is duplicated by the current-project selection fixtures that landed in main via #627, which contains #631's versions of the same scenarios with the same expected unit sets.