Compress CAS blobs and configure output symlink capture

Summary

• Add zstd-backed blob storage in the local CAS while keeping BLAKE3 digests based on uncompressed bytes.
• Preserve compatibility with existing raw blobs and report logical blob sizes for compressed entries.
• Add directory-output symlink modes so script caches materialize external symlink targets by default and can opt into preserving links with # once output-symlinks "preserve".
• Document the new script annotation and add tests for compression, v1 directory restore, symlink handling, and action key partitioning.

Testing

• mise exec -- cargo test -p once-cas -p once-core -p once-cli -p once-frontend
• mise exec -- cargo clippy -p once-core -p once-cli -p once-frontend --all-targets -- -D warnings
• mise exec -- cargo fmt --all -- --check
• git diff --check

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