Migrate selected models to spec v1

[0xShug0]

Summary

• Migrate Demucs, Mel-Band RoFormer, HeartMuLa, Hviske ASR, and SeedVC toward schema-v1 model specs.
• Remove thin loader files for migrated models where the spec-backed loader path can be used directly.
• Add/standardize option compatibility handling so old request option names can continue to work during migration.
• Update model docs and path/perf cases to reflect the normalized option surface.

Validation

• Debug builds were run during the migration pass.
• Migrated model path tests were run for the touched families during their individual migration steps, including SeedVC CUDA validation with legacy-option path cases and explicit unknown-option rejection.

Notes

• This PR keeps model_specs_v1/ read-only as reference material.
• Existing GGUFs that embed legacy specs continue to load through the installed runtime contract, with explicit warning behavior for legacy embedded specs.