The download pipeline had grown into one large file that mixed sentinel
transaction handling, peer planning, transport, retry, and top-level
orchestration. Split it into a download/ module tree with one file per
concern so future lifecycle changes can be reviewed at the right boundary.
The public crate surface remains download::download_game_files. Helper types
and functions are kept pub(super) or private so the refactor does not widen
the API or encourage new callers to depend on internals. The version.ini
transaction helpers stay local to version_ini.rs; the proposed fs_util
extraction is intentionally left for the later atomic-index work, where a
second caller exists.
There is no intended runtime behavior change.
Test Plan:
- just fmt
- just test
- just clippy
- just build
Refs: none
