feat: constrain Nemotron automatic language detection

[luantak]

Summary

Add a generic allowed_languages run option for automatic Nemotron language detection. The RNNT decoder masks language-control token IDs outside the allowlist at every selection step, including later streaming steps. An empty list leaves automatic detection unrestricted; an explicit language keeps the existing forced-language path.

Scope

• Append the option to the public C run parameters and regenerate ABI declarations for Rust, Python, and TypeScript.
• Resolve advertised language codes to token IDs and apply the mask only to language-control tokens.
• Pass the option through the safe Rust wrapper and preserve streaming parameter lifetimes.
• Add focused decoder, API, stream validation, and optional real-model streaming tests.
• Dependent Handy integration: feat: constrain automatic language detection from the language picker cjpais/Handy#2138.

AI Assistance

Codex assisted with the implementation. I tested this e2e with my handy app checkout locally.

Validation

• scripts/ci/clang-format.sh --check passed.
• Focused CTest targets passed: API smoke, language-mask unit, and Parakeet stream validation.
• transcribe_nemotron_allowlist_real_stream passed with the Nemotron 3.5 ASR Streaming Q8_0 GGUF.
• Rust safe-wrapper library and no-model tests passed against the local native build.
• Binding generators and ABI checks passed.
• The complete CTest suite still needs to run after all test targets are built; the current local build contains only the targeted binaries.