Upgrade Melody pet to Codex v2

[yousizaitianqiong]

Summary

Closes #1.

This PR upgrades the Melody Codex pet package to the current v2 sprite format and includes validation artifacts for review.

Changes

• Updates pet.json with spriteVersionNumber: 2.
• Replaces spritesheet.webp with a v2 1536x2288 atlas using an 8 x 11 grid and 192 x 208 cells.
• Adds row 9 and row 10 as 16 looking-direction frames.
• Adds alpha=1 hit-fill to used cells so the overlay hit bounds stay stable after Codex restarts.
• Updates preview.webp and rebuilds both package zip files.
• Adds qa-v2-hit-stable contact sheet, looking-direction GIF, validation JSON, and a validation script.
• Refreshes README documentation for the v2 package.

Validation

• python scripts/validate_melody_v2_hit_stable.py spritesheet.webp qa-v2-hit-stable
• Confirmed atlas size: 1536x2288
• Confirmed grid: 8 x 11
• Confirmed cell size: 192x208
• Confirmed 16 non-empty looking-direction frames in rows 9 and 10
• Confirmed hit-fill coverage for used cells