feat(server): client_mss_bulk — raise MSS after handshake to cut pps

client_mss (e.g. "tspu", MSS=92) fragments the whole connection to evade
DPI on the ServerHello, but it also fragments bulk payload, multiplying
outgoing packets-per-second ~10x. On hosts whose abuse detection counts
pps (not bandwidth) this trips packet-flood limits.

Add an optional [server].client_mss_bulk: keep the low client_mss for the
handshake (ServerHello stays fragmented => DPI bypass intact), then raise
the client socket MSS to client_mss_bulk once the connection enters the
post-handshake (bulk transfer) phase, so bulk data uses normal-size
segments and pps drops back to normal. Same preset/int grammar as
client_mss. Opt-in: when unset, the handshake MSS is kept for the whole
connection (unchanged behavior).

Linux-only (setsockopt TCP_MAXSEG via raw fd, mirroring TCP_USER_TIMEOUT);
no-op on other unix. Documented in CONFIG_PARAMS.{en,ru}.

src/config/load.rs

@@ -300,6 +300,7 @@ const SERVER_CONFIG_KEYS: &[&str] = &[
     "listen_unix_sock_perm",
     "listen_tcp",
     "client_mss",
+    "client_mss_bulk",
     "proxy_protocol",
     "proxy_protocol_header_timeout_ms",
     "proxy_protocol_trusted_cidrs",