Remove production panics and expects from libtortillas runtime paths

[artrixdotdev]

Problem

A long-running TUI cannot crash because a peer disconnects, metadata parse fails, storage config is invalid, or current directory lookup fails. Several runtime paths still use panic!, unwrap, expect, or assertions for recoverable failures.

Examples from review:

• crates/libtortillas/src/engine/mod.rs:176
• crates/libtortillas/src/torrent/messages.rs:104
• crates/libtortillas/src/torrent/messages.rs:169
• crates/libtortillas/src/peer/actor.rs:247
• crates/libtortillas/src/peer/actor.rs:628
• crates/libtortillas/src/peer/actor.rs:637

Dependency

Blocked by #231 because recoverable failures should map into typed public/internal errors.

Acceptance criteria

• Recoverable runtime failures return errors or emit typed error events instead of panicking.
• Network send/receive failures do not crash actors unnecessarily.
• Invalid frontend operations return typed invalid-state errors.
• Tests cover at least the high-risk panic replacements.
• Remaining unwrap/expect usages are limited to tests, invariants that are provably impossible, or documented unreachable states.