Replace NAVA special-case backend with generic diffusion components

[AlexEisie]

Summary

• replace the old NAVA-specific backend with a generic task backend/component structure
• support NAVA LoRA through the diffusion backend and nava component instead of --task nava or yeto/nava
• remove downstream smoke/data-prep scripts and S3/pre-download coupling from Yeto core
• keep non-NAVA main updates by basing this cleanup branch on current origin/main

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m compileall -q yeto
• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=. pytest -q -p no:cacheprovider ... -k "not upload_sky and not cycle" — 243 passed, 2 deselected
• cargo test --manifest-path syncer/Cargo.toml — 31 passed
• git diff --check

Notes

• This PR is intentionally based on current main so a normal merge removes the old NAVA-special-case files without force-pushing.
• The working tree matches integrate/non-nava-main-updates after applying the cleanup diff.