docs: OpenUtau .oudep install guide (old-API → v0.1.0 old_ pkg) + fix stale paths

[KakaruHayate]

Docs refresh:

1. OpenUtau integration guide (README.md + README_CN.md): how to pick the right .oudep — current (new serve-API protocol) from the latest release, early/old-API OpenUtau builds → the old_-prefixed packages from the v0.1.0 release only (no longer silently pointing old builds at the newest package).
2. BUILDING.md serve-mode section: documents the current serve protocol location + the old-API/old_-v0.1.0 caveat.
3. Stale paths: the repo was flattened/renamed, but READMEs + scripts/external_consumer examples still referenced ggml_backend/assets|build|scripts, which no longer exist. Updated all to the current root layout (build/, scripts/, game_*.gguf).

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📥 Commits

Reviewing files that changed from the base of the PR and between 97f9277 and 0862fc2.

📒 Files selected for processing (5)

• BUILDING.md
• README.md
• README_CN.md
• examples/external_consumer/README.md
• scripts/README.md

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