test(peer-cli): add large exact-transfer coverage

Add deeper peer CLI coverage for file-transfer integrity and multi-peer
chunking. The alpha fixture now carries a real renamed RAR archive larger
than 100 MB for alienswarm, which gives the chunk planner enough work to
split a single game archive across multiple peers.

Expose completed chunk source details as a peer event and have the CLI print
that event as JSONL. This keeps transfer behavior in lanspread-peer while the
CLI remains a harness that reports what the peer runtime did. The Tauri shell
logs the event at debug level so the shared PeerEvent enum stays exhaustive.

Document the new S13/S14 scenarios and record the manual run evidence,
including SHA-256 manifests and the per-peer byte split for the large archive.

Test Plan:
- just fmt
- just test
- just peer-cli-build
- just clippy
- just peer-cli-image
- unrar t -idq crates/lanspread-peer-cli/fixtures/fixture-alpha/alienswarm/alienswarm.eti
- Manual peer CLI: bravo -> deep-small-client bfbc2 download with matching SHA-256 manifests
- Manual peer CLI: alpha -> deep-stage-b alienswarm download with matching SHA-256 manifests
- Manual peer CLI: alpha + deep-stage-b -> deep-stage-c alienswarm download with chunk events from both peers and matching SHA-256 manifests

Refs: PEER_CLI_SCENARIOS.md S13 S14

crates/lanspread-peer/src/lib.rs

@@ -91,6 +91,14 @@ pub enum PeerEvent {
     },
     /// Download has started for a game.
     DownloadGameFilesBegin { id: String },
+    /// A file chunk has been downloaded from a peer.
+    DownloadGameFileChunkFinished {
+        id: String,
+        peer_addr: SocketAddr,
+        relative_path: String,
+        offset: u64,
+        length: u64,
+    },
     /// Download has completed successfully.
     DownloadGameFilesFinished { id: String },
     /// Download has failed.