Improve OSS readiness and maintainer workflow

[Firton]

Summary

This PR improves Ultra-TTS as an early open-source local TTS GUI and CLI workspace.

Changes

• Rewrites README for clearer OSS positioning and setup guidance
• Adds SECURITY.md, CONTRIBUTING.md, ROADMAP.md, and CHANGELOG.md
• Adds lightweight GitHub Actions test workflow
• Adds issue templates, PR template, and CODEOWNERS
• Adds Codex for Open Source application draft
• Adds docs placeholders for future screenshots and audio samples
• Updates .gitignore to keep generated audio, models, logs, caches, and virtual environments out of Git
• Adds lightweight deterministic tests
• Generalizes machine-specific wording

Validation

• git diff --check
• python -m unittest discover -s tests
• lightweight Python compile check

Notes

This PR does not claim adoption, external usage, production usage, downloads, or broad maturity. Ultra-TTS is intentionally described as an early OSS project focused on local Japanese and multilingual TTS workflows.

Follow-up after merge

• Set GitHub About description and topics
• Create v0.1.0 - Initial public OSS release
• Add screenshots and audio samples when ready
• Consider enabling branch protection and requiring the tests workflow