isV2LayoutValid

For a v2 / hybrid torrent, checks the BEP-52 layout invariant that every file (except the last) begins on a piece boundary. Pad files in a hybrid v1 list, and the v2 file tree's natural per-file trees, are supposed to enforce this. Returns true for v1-only torrents (the invariant does not apply) and for an empty/degenerate layout.

This is a non-throwing guard: callers can decide whether to refuse a malformed torrent; parsing itself never crashes on a misaligned file. Mirrors the alignment checks libtorrent performs while reconstructing a v2 file_storage.