Fix CLI version command and RMVPE fallback

[Camier]

Summary

• ensure rwc --version always resolves the package version without requiring an installed distribution
• keep the VoiceConverter.use_rmvpe flag aligned with the user request while tracking RMVPE availability for runtime fallbacks
• warn and gracefully fall back to CREPE pitch extraction when RMVPE assets are missing

Testing

• pytest

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Codex Task

[Copilot]

Pull Request Overview

This PR fixes two issues: ensuring the CLI's --version command always displays the package version correctly, and improving RMVPE (pitch extraction model) fallback behavior to maintain the user's preference while gracefully handling missing model files.

• Adds explicit version and program name to CLI's @click.version_option decorator
• Introduces rmvpe_available flag to track RMVPE model availability separately from user preference
• Adds warning when RMVPE is requested but unavailable, with automatic fallback to CREPE

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rwc/cli/init.py	Imports __version__ and passes it explicitly to @click.version_option to ensure version display works without requiring installed distribution
rwc/core/init.py	Separates RMVPE user preference (use_rmvpe) from availability (rmvpe_available), adds warning for missing RMVPE model, and updates pitch extraction method selection logic

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