[codex] Add README inline audio players

[kymuco]

Summary

• Added MP3 versions of the four short synthetic demo audio examples under docs/assets/demo/audio/.
• Updated the README demo audio section with inline <audio controls preload="none"> players.
• Kept regular Markdown fallback links for both MP3 and existing OGG files.

Notes

• This is a docs/demo-assets update only.
• No autoplay is used.
• Inline player src values use raw.githubusercontent.com/kymuco/practicelens/main/... so the rendered README receives audio bytes after merge.
• Existing OGG files were preserved.
• MP3 files add 21,992 bytes; total docs/assets/demo/ size is 62,643 bytes (~61.2 KiB).
• Demo audio remains synthetic/generated; no private or real audio was added.
• No scoring, alignment, preprocessing, CLI/API contracts, schemas, or tests were changed.

Validation

• ruff check . passed.
• python -m build passed.
• pytest tests completed with 93 passed and 3 failed on Windows. The failures are the existing Windows path-separator issue tracked in [Tests] Normalize path separators in session output assertions on Windows #105, not caused by this docs/assets update.