Allow duplicate DropPoint filenames

[shunichironomura]

Summary

• Disambiguate duplicate DropPoint manifest filenames case-insensitively using numbered suffixes.
• Preserve manifest order and payload slicing while allowing generated-name collisions to advance to the next suffix.
• Add unit coverage for duplicate filename imports.

Fixes #296

Tests

• just check-fmt
• just check-clippy
• just check-vp (passes with existing unicorn/no-empty-file warning in src/lib/index.ts)
• just check-frontend
• just test