feat(sync): add deterministic directory manifest model

[rldyourmnd]

Summary

• add bounded canonical file/dir/symlink directory snapshot model
• validate normalized paths, duplicate entries, symlink target limits and snapshot roots
• add deterministic diff and exact identity-based rename candidates
• document that filesystem scanning and recursive wire behavior remain separate gates

Validation

• cargo fmt --check
• cargo test -p rds-sync
• cargo clippy -p rds-sync --all-targets -- -D warnings