Fail pack on corrupt delta base

[peters]

Summary

• classify restored artifact SHA-256 mismatches as integrity failures instead of generic storage errors
• fail surge pack when delta base restoration detects stored full-archive corruption and clear staged artifacts before returning
• add regression coverage for both the direct corrupt-base case and the reconstructable older-chain case

Why

A stored full archive whose bytes no longer match its recorded SHA-256 is a data-integrity problem in release storage. Packaging should stop and force remediation instead of silently routing around the corrupt blob and publishing a new delta on top of an unhealthy base path.

Impact

surge pack now fails closed for base-artifact integrity mismatches while preserving the existing non-fatal behavior for ordinary delta-build failures.

Validation

• ./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