perf(peer): widen LAN bulk-transfer windows and buffers

Centralize the bulk-transfer sizing in config.rs and bump the values used
on both ends of a QUIC connection:

- CHUNK_SIZE: 32 MiB -> 128 MiB
- QUIC_CONNECTION_DATA_WINDOW: 64 MiB -> 256 MiB
- QUIC_STREAM_DATA_WINDOW: 32 MiB -> 128 MiB
- QUIC_MAX_SEND_BUFFER_SIZE: 32 MiB -> 128 MiB
- QUIC_INITIAL_CONGESTION_WINDOW: 1 MiB -> 4 MiB
- FILE_TRANSFER_BUFFER_SIZE: 64 KiB -> 1 MiB (new constant)

The previous 32 MiB stream window was already comfortably above the
bandwidth-delay product of a sub-millisecond LAN at 2.5 GbE. The further
bump is deliberately generous: the goal is to push flow control and
per-syscall overhead far enough out of the way that they cannot be the
suspect when isolating the remaining LAN download bottleneck (disk, NIC,
or s2n-quic platform offload on the sending host). Memory pressure from
the larger windows is not observable on a desktop client moving GB-sized
games.

stream_file_bytes previously read the local file in 64 KiB chunks. At
multi-Gbit/s send rates that produced many thousands of disk reads per
second; bumping to 1 MiB keeps the per-file syscall load modest with no
measurable latency cost on streamed bulk transfers. The buffer size lives
in config.rs as FILE_TRANSFER_BUFFER_SIZE so it stays adjustable from one
place.

Also add a started/MiB-per-second log line at info level when a file
finishes streaming. This matches the S37 measurement methodology already
used in the peer-cli harness and makes per-file send throughput visible in
normal operation.

The peer-cli extended-scenarios harness uses CHUNK_SIZE as the tolerance
bound for chunk-boundary variance in its assertions, so its constant is
bumped to match. The multi-chunk planning unit test is rewritten to
reference CHUNK_SIZE symbolically (CHUNK_SIZE * 3 + CHUNK_SIZE / 2)
instead of a hardcoded 120 MiB; the previous literal would silently
degrade into a single-chunk test at the new chunk size and stop
exercising the spread-across-peers code path.

Test Plan:
- just fmt
- just clippy
- just test
- python3 crates/lanspread-peer-cli/scripts/run_extended_scenarios.py S37 \
  --build-image
- python3 crates/lanspread-peer-cli/scripts/run_extended_scenarios.py S37

Refs: local LAN download performance investigation on 2026-05-20.
Depends-on: d7f7dc737e (QUIC UDP socket buffer sizing).

crates/lanspread-peer/src/download/planning.rs

@@ -247,7 +247,7 @@ mod tests {
     fn build_peer_plans_spreads_large_file_chunks_across_shared_peers() {
         let peers = vec![loopback_addr(12000), loopback_addr(12001)];
         let large_file = "game/large.eti";
-        let file_size = 120 * 1024 * 1024;
+        let file_size = CHUNK_SIZE * 3 + CHUNK_SIZE / 2;
         let mut file_peer_map = HashMap::new();
         file_peer_map.insert("game/version.ini".to_string(), peers.clone());
         file_peer_map.insert(large_file.to_string(), peers.clone());