[chrischeng-c4]

Closes #17

Summary of Changes

• Implemented SetKeyLayout and SetKeyError in crates/service-backup/src/domain/set_key_layout.rs providing immutable, collision-safe key derivation (artifact_key, manifest_key, latest_key, set_prefix) and parse_set_id with strict validation of prefix, set_id, and artifact name.
• Implemented SetInventory, SetState, RetentionPlan, and plan_retention in crates/service-backup/src/domain/retention_plan.rs providing deterministic retention planning, keeping the newest keep_latest complete sets (at least 1), marking older complete sets for deletion, and classifying aged partial sets as orphans based on orphan_grace.
• Declared modules in crates/service-backup/src/domain.rs and re-exported public types from crates/service-backup/src/lib.rs.
• Registered and implemented comprehensive unit tests covering all 10 acceptance scenarios in crates/service-backup/tests/it/set_layout.rs.
• Fixed needless as_bytes lint warning in crates/service-backup/tests/it/projected_admin_transport.rs.

Local Validation

• Unit tests: cargo test -p service-backup --test it set_layout (10 passed, 0 failed)
• Formatting: cargo fmt -p service-backup -- --check (clean)
• Lints: cargo clippy --no-deps -p service-backup --all-features --all-targets -- -D warnings (clean)

Cloud CI Validation Needed (NOT RUN locally per policy)

• MR CI: affected unit and functional/integration checks across cloud environments (GCS/S3 sinks).
• Performance regression checks and full release qualification matrix.