Optimize sparse delta apply path

[peters]

Summary

• apply sparse-file-ops deltas directly to a staged filesystem tree restored from the pristine base package instead of round-tripping through an intermediate rebuilt archive during update materialization
• keep the rebuilt latest full archive warm in the local artifact cache after delta application
• add regression coverage for direct sparse delta directory application, persistent-asset divergence, and preserved directory modes

Why

A recent Linux x64 update spent most of its time preparing a tiny sparse delta against a cached full package.

The hot path was not network transfer or app shutdown. It was local archive work:

1. load the cached full archive
2. expand it to a temporary tree
3. apply sparse file ops
4. repack a new full archive
5. extract that rebuilt archive again into the staged app tree

That makes sparse deltas pay full-archive costs even when the delta payload itself is very small.

Root Cause

UpdateManager treated sparse file deltas like archive deltas all the way through staging. The updater rebuilt a full archive in memory and then extracted it, even though the sparse delta format already describes filesystem operations.

What Changed

• added a direct sparse apply path in UpdateManager that:
  • restores the pristine base full archive into the staged tree
  • applies decoded sparse file ops directly to that staged tree
  • repacks once at the end for integrity verification and cache warming
• preserved the existing archive-based path for non-sparse delta formats
• retained the installed-app fallback archive reconstruction path only for cases where no base full archive can be restored
• cached the rebuilt latest full archive after delta application so the next update can reuse it
• added regressions covering:
  • persistent assets that intentionally diverge from packaged contents
  • unchanged non-default directory modes during sparse apply

Impact

For sparse deltas, this removes an unnecessary archive round-trip from the apply path while preserving the packaged archive basis required for persistent assets and hash-stable directory metadata.

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