Fix CosyVoice3 Metal flow layout

[0xShug0]

Summary

• Materialize the CosyVoice3 pre-lookahead conditioning tensor after transpose before ggml_pad_ext() on Metal.
• Reuse the original token embedding as the residual instead of rebuilding the same embedding node.
• Remove the temporary CosyVoice3 Metal backend block.

Validation

Built audiocpp_cli from build/debug, then ran the CosyVoice3 official path case against this branch and a fresh main baseline.

Backend	Path case	Result	Compare
CUDA	cosyvoice3_official_paths	Pass	artifacts/text match, 0 difference(s)
Vulkan	cosyvoice3_official_paths	Pass	artifacts/text match, 0 difference(s)

Timing movement for the short path case was small: CUDA +1.87%, Vulkan +1.78%.