Add MiniMax Cloud TTS engine (minimax-cloud)

[octo-patch]

Summary

This PR adds MiniMax as a first-class cloud text-to-speech engine (minimax-cloud), joining the existing lineup of Google Cloud, Azure, Amazon Polly, OpenAI Cloud, ElevenLabs, and Deepgram.

What's included

• src/synthesis/MiniMaxCloudTTS.ts — new engine module:

  • Calls POST https://api.minimax.io/v1/t2a_v2 (non-streaming, returns hex-encoded MP3)
  • Supports speech-2.8-hd (default) and speech-2.8-turbo models
  • 12 verified voice IDs (English and bilingual English/Chinese)
  • Speed clamped to [0.5, 2.0] as required by the API
  • API key via miniMaxCloud.apiKey option or MINIMAX_API_KEY environment variable

• src/api/Synthesis.ts — integrated into the synthesis pipeline:

  • 'minimax-cloud' added to SynthesisEngine union type
  • miniMaxCloud?: MiniMaxCloudTTSOptions in SynthesisOptions
  • Synthesis dispatch (case 'minimax-cloud')
  • Voice list dispatch
  • synthesisEngines metadata entry

• src/tests/MiniMaxCloudTTS.test.ts — 13 unit tests using Node.js built-in node:test:

  • Voice list structure validation
  • Default option values
  • TypeScript interface compliance
  • Error handling (missing API key)
  • Engine registration in synthesisEngines
  • All tests pass without a network call or API key

• docs/Engines.md and README.md — documentation updated

Usage

import * as API from 'echogarden'

const result = await API.synthesize('Hello from MiniMax!', {
  engine: 'minimax-cloud',
  miniMaxCloud: {
    apiKey: process.env.MINIMAX_API_KEY,
    model: 'speech-2.8-hd',
  },
  voice: 'English_Graceful_Lady',
  language: 'en-US',
})

Or set MINIMAX_API_KEY in the environment and omit apiKey.

Available voices

Voice ID	Language(s)	Gender
English_Graceful_Lady	en	female
English_Insightful_Speaker	en	male
English_radiant_girl	en	female
English_Persuasive_Man	en	male
English_Lucky_Robot	en	male
Wise_Woman	en, zh	female
cute_boy	en, zh	male
lovely_girl	en, zh	female
Friendly_Person	en, zh	unknown
Inspirational_girl	en, zh	female
Deep_Voice_Man	en, zh	male
sweet_girl	en, zh	female