refactor(core): split release restore planning and artifact recovery

[peters]

Summary

• split releases::restore into focused planning, candidate, recovery, and retention modules
• keep the root restore module orchestration-focused and remove its maintainability baseline entry
• add a cleanup rollout document that records completed phases, remaining oversized files, and the serial squash-merge workflow

Behavior Impact

• no intended behavior change to restore planning, delta reconstruction, cache-aware restore selection, or artifact retention
• keeps the existing restore test coverage in the root module while moving implementation details into leaf modules
• updates the maintainability baseline to reflect that crates/surge-core/src/releases/restore.rs is now below the 600-line target

Test Evidence

• cargo check -p surge-core
• cargo test -p surge-core releases::restore
• cargo clippy -p surge-core --all-targets --all-features -- -D warnings -W clippy::pedantic
• ./scripts/check-maintainability.sh
• ./scripts/sync-surge-core-vendor.sh --check
• ./scripts/check-version-sync.sh
• cargo fmt --all -- --check
• RUSTFLAGS="-D warnings" cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• dotnet format dotnet/Surge.slnx --verify-no-changes
• dotnet test dotnet/Surge.slnx --configuration Release