RUST-FEAT-001 — eD2K — Implement client UDP source reask and queue-slot persistence

[itlezy]

Part of the post-0.7.3 suite forward program (Phase 0). Workflow state is tracked here; the spec lives in the linked doc.

Spec: https://github.com/emulebb/emulebb-rust/blob/main/docs/active/items/RUST-FEAT-001.md
Roadmap: emulebb-tooling docs/active/SUITE-JOINT-ROADMAP.md