Prepare for v0.5.0: Fix public API return types (Bytes → Uint8Array)

[Copilot]

The Bytes conditional type was incorrectly defined (comparing against string "undefined" and referencing the Buffer constructor rather than instance type), causing public API signatures to expose an environment-dependent type instead of a consistent Uint8Array.

Changes

• src/types.ts: Removed the broken Bytes conditional type entirely; only IS_BUFFER_SUPPORTED remains
• src/index.ts: encrypt and decrypt return types changed from Bytes → Uint8Array
• src/keys/PrivateKey.ts: secret getter return type changed from Bytes → Uint8Array; removed stale // TODO: Uint8Array comment
• src/keys/PublicKey.ts: uncompressed and compressed getter return types changed from Bytes → Uint8Array

At runtime, Node.js environments still return Buffer (a Uint8Array subtype) for backwards compatibility, but the TypeScript contract is now consistently Uint8Array across all environments.

// v0.4.x — environment-dependent type, broken in browser TS projects
encrypt(...): Bytes  // Buffer | Uint8Array depending on conditional

// v0.5.0 — consistent public contract
encrypt(...): Uint8Array  // Buffer at runtime in Node (still assignable)

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[kigawas]

Close this pr