Make frontend-owned output and storage policy explicit

[artrixdotdev]

Problem

Engine::new defaults the output path to std::env::current_dir(), which is application policy inside the library. A TUI should explicitly own where downloads go, how defaults are chosen, and how storage configuration is presented to users.

Relevant code:

• crates/libtortillas/src/engine/mod.rs:171
• crates/libtortillas/src/torrent/storage.rs

Dependency

Blocked by #222 because storage/output policy should be part of the frontend facade contract.

Acceptance criteria

• Library defaults are safe and clearly documented.
• Frontends can explicitly configure output path and storage strategy through stable API.
• Relative path behavior is documented and tested.
• Invalid output/storage combinations return typed errors, not panics.
• This issue coordinates with Harden torrent output path sanitization #234 for path safety.