Migrate BS-RoFormer to model spec v1

[mirek190]

Summary

Migrates BS-RoFormer to the current schema-v1 model-spec architecture described in #128 without changing the shared Mel-Band RoFormer interface.

• makes model_specs/bs_roformer.json the single source of truth with schema_version: 1, typed metadata/options, package/UI hints, and GGUF/safetensors resources
• removes the duplicate model_specs_v1/bs_roformer.json
• moves only make_bs_roformer_loader() to the generic spec-backed loader path beside the shared session
• passes the BS-RoFormer ModelContract into shared RoFormer session construction and validates bs_roformer.* session keys against it
• keeps bs_roformer.weight_type and bs_roformer.num_overlap as the public namespaced session interface
• deliberately retains the existing custom Mel-Band RoFormer loader and its current behavior
• does not change RoFormer graphs, weights, overlap-add math, Flash Attention, or output samples

Shared-loader handling

BS-RoFormer and Mel-Band RoFormer share assets/runtime/session source files. This migration is additive: the generated registry includes the existing Mel-Band loader factory from loader.h and the new BS schema-backed factory from session.h. The old BS factory is removed from the custom loader, avoiding duplicate registration while leaving Mel-Band intact.

Validation

• based on current main (c0d1c9f)
• Windows MSVC Release custom composite build: AUDIOCPP_MODEL_SET=custom, AUDIOCPP_MODELS=bs_roformer, target audiocpp_cli
• existing standalone BS-RoFormer ep368 Q8 GGUF inspection: passed; v1 metadata and both session options exposed
• real 8-second stereo CPU separation with Q8 GGUF and bs_roformer.num_overlap=1: passed; generated vocals.wav and instrumental.wav in 20.120 s session time
• existing Mel-Band RoFormer Q8 GGUF inspection: passed; only its unchanged mel_band_roformer.weight_type interface is exposed
• loader catalog self-tests: passed
• loader/catalog synchronization: passed with the pre-existing legacy model-manager warning for BS-RoFormer
• git diff --check: passed

No model output or performance behavior is intentionally changed by this migration.

[0xShug0]

Thanks @mirek190, all three PRs have been merged!