deprecate/remove `base64` module once native `Uint8Array.toBase64` lands unflagged in Node.js

[knoan]

goal

Remove the @metreeca/core/base64 module (or deprecate then remove) once V8 / Node.js expose the native
Uint8Array.prototype.toBase64 / Uint8Array.fromBase64 APIs unflagged, so consumers can use the standard APIs
directly.

scope

• Monitor Node.js releases for the version where the feature becomes default-on (currently gated behind the V8
  --js-base-64 experimental flag through Node 24.x; already Baseline 2025 in evergreen browsers).
• Bump engines.node in package.json to the minimum Node.js version that ships the feature unflagged.
• Delete src/standard/base64.ts and src/standard/base64.test.ts, or mark encodeBase64 / decodeBase64
@deprecated with migration guidance pointing at the native API, then remove in a later major.
• Drop the ./base64 export from package.json.
• Remove the row and the description-list reference from README.md.
• Log the change under CHANGELOG.md → Removed / Deprecated.

acceptance criteria

• engines.node raised to the first version that ships Uint8Array.toBase64 / fromBase64 unflagged
• src/standard/base64.ts removed, or marked @deprecated with migration snippet
• ./base64 export removed from package.json (when fully removed)
• README.md updated
• CHANGELOG.md entry added under [Unreleased]

dependencies

• V8 unflagging the --js-base-64 experimental flag
• Node.js release shipping the unflagged V8
• Willingness to raise the minimum Node.js version

notes

Current wrapper exists only because the native path is flagged on Node; browsers have been Baseline since
September 2025 (Chrome 140, Firefox 133, Safari 18.2). Native migration target:

function encodeBase64(plain: string): string {
    return new TextEncoder().encode(plain).toBase64({ alphabet: "base64url", omitPadding: true });
}
function decodeBase64(encoded: string): string {
    return new TextDecoder().decode(Uint8Array.fromBase64(encoded, { alphabet: "base64url" }));
}

Project baseline to watch alongside this: tsconfig.json target: "ES2022", module: "NodeNext" — likely needs a
lib bump once TypeScript ships types for toBase64 / fromBase64 in its default lib. See the > [!NOTE] panel at
the top of src/standard/base64.ts for the full background.