fix: tighten Swift CLI command surface

[hankbobtheresearchoor]

Summary

Tightens the Swift provider CLI surface based on local smoke-test findings:

• adds a real darkbloom serve foreground command
• rejects unknown top-level subcommands instead of printing top-level help with exit 0
• validates darkbloom-enclave attest --pub-key as base64-encoded 32-byte X25519 input
• clarifies that darkbloom-enclave wallet-address is ephemeral for the stateless helper
• fixes the local metallib README example

Validation

• swift build --product darkbloom -c debug
• swift build --product darkbloom-enclave -c debug
• swift test --filter attestationInputValidatorRequiresBase64X25519PublicKey
• DARKBLOOM_NO_UPDATE_CHECK=1 .build/debug/darkbloom serve --help
• DARKBLOOM_NO_UPDATE_CHECK=1 .build/debug/darkbloom foobar --help exits 64
• DARKBLOOM_NO_UPDATE_CHECK=1 .build/debug/darkbloom-enclave attest --pub-key not-base64 exits non-zero
• DARKBLOOM_NO_UPDATE_CHECK=1 .build/debug/darkbloom-enclave attest --pub-key <32-byte-base64> succeeds
• DARKBLOOM_NO_UPDATE_CHECK=1 .build/debug/darkbloom-enclave wallet-address --help

[vercel]

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