Cap DiffSinger vocoder num_mel_bins at 255

[KakaruHayate]

Problem

The DiffSinger vocoder currently loads any onnx model that exposes a mel input plus optional f0, with no bounds checking on its declared configuration. As a result, a malformed voicebank can ship a vocoder slot that points at a non-vocoder onnx model (for example, a DiffSinger acoustic model masquerading as a vocoder, or some other graph that requires an extra-large mel tensor to drive its internal logic).

Real DiffSinger vocoders in the wild use 80 or 128 mel bins.

Fix

Add a hard-coded upper bound DsVocoderConfig.MaxMelBins = 255 and reject both the vocoder and the acoustic-model dsconfig when num_mel_bins falls outside [1, 255].

The check fires in two places:

• DsVocoder constructor: rejects the voicebank at load time with the same MessageCustomizableException other vocoder load failures use, so the entry never reaches the onnx session.
• DiffSingerRenderer.InvokeDiffsinger: redundant runtime check on both the vocoder and the acoustic model cfg, alongside the existing mel_base / mel_scale validations, as defense in depth.

The threshold is intentionally hard-coded rather than a yaml field: lowering it is a deliberate code change, not something a voicebank author can override.

[HuanLinOTO]

Review: LGTM with nits

整体评价：功能目标清晰，安全加固方向正确，双层检查设计合理，阈值选择恰当（255）。同意合并，但提几个小建议：

建议修复

1. 错误消息措辞不统一

• DsVocoder 构造器：got {config.num_mel_bins}
• InvokeDiffsinger：but got {vocoder.num_mel_bins}
  建议统一为 but got，方便 grep。

2. 边界条件可改为 <= 0
当前用 < 1，逻辑等价但语义可以更清晰。num_mel_bins 是 int 且默认 128，正常情况下不会为 0，<= 0 更直观：

if (config.num_mel_bins <= 0 || config.num_mel_bins > MaxMelBins)

建议补充

3. 建议补充单元测试
此类安全边界检查非常适合单元测试。建议覆盖：

• num_mel_bins = 0 或负数 → 应拒绝
• num_mel_bins = 256 → 应拒绝
• num_mel_bins = 1 和 255 → 应接受（边界合法值）

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Commit: 0ed2251
Files: DiffSingerVocoder.cs / DiffSingerRenderer.cs (+27 lines)

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[yqzhishen]

This check also prevents vulnerabilities like extremely high mel bins (the only configurable dynamic dimension) that will explode the user's disk.