perf(cache): split Swift actions and prefetch output restore

Summary

• Split apple_library Swift compilation into separate module/header and static archive actions so cache keys can track Swift interface outputs independently from archive packaging.
• Compose graph target action digests from every declared action for multi-action targets, preserving single-action digests unchanged while making downstream cache keys reflect provider-visible outputs.
• Prefetch cached output blobs concurrently during restore with a bounded window and disk staging, preserving deterministic final materialization without retaining every blob in memory.
• Added regression coverage for multi-output restore, prefetch windows, failure-before-materialization behavior, target digest aggregation, and the Apple Swift action split.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test -p once-core outputs::tests
• mise exec -- cargo test -p once-cli commands::graph::analysis::tests
• mise exec -- cargo test -p once-frontend --lib
• mise exec -- cargo clippy -p once-core -p once-cli -p once-frontend --all-targets -- -D warnings
• xcrun --sdk iphoneos swiftc -module-name AppCore -target arm64-apple-ios17.0 -parse-as-library -static -emit-module -emit-module-path <tmp>/AppCore.swiftmodule -emit-objc-header -emit-objc-header-path <tmp>/AppCore-Swift.h fixtures/apple_library/apps/ios/AppCore/Sources/Greeting.swift

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