webrtc: fix race edge cases that discard or mislabel a direct connection

Three correctness fixes in the transport race, plus three convention
cleanups.

- race_transports_prefer_webrtc committed a relayed result while a direct
  attempt was still in flight: the others arm returned on
  webrtc_fut.is_none() even with an unfinished direct future, and the
  WebRTC-error arm returned a held relay without checking others_fut. A
  relay is now committed only when nothing direct can still arrive (or
  the window expires); a parked relay is also preferred over composing
  an error when both sides fail. Three regression tests, mutation-checked.

- connect()'s plain select_ok let a TURN-relayed WebRTC win as "first
  success", dropping still-racing UDP/IPv6 direct attempts and reporting
  the relayed pair as direct. It now runs through the same prefer-P2P
  race with each attempt carrying whether its path is direct, and the
  WebRTC future resolves is_relayed() so a TURN win is held behind
  direct attempts, not committed as one.

- The RelayResponse path kept direct == true when a WebRTC win's DTLS
  handshake failed and it fell back to relay, so the relay was reported
  P2P. Clear the flag with the transport switch.

- Trim the OffererGuard doc to the three-line max; move the new
  enable-webrtc localization key to the end of every lang list; the KCP
  option constant moved to hbb_common config::keys (0f663aa).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ExUfAkYbq8UC9pQCiLy8TQ

src/common.rs

@@ -2452,8 +2452,6 @@ pub fn is_udp_disabled() -> bool {
     Config::get_option(keys::OPTION_DISABLE_UDP) == "Y"
 }
 
-pub const OPTION_ENABLE_KCP_CC: &str = "enable-kcp-congestion-control";
-
 /// Run KCP with its congestion window (nc=0) instead of the turbo profile it has always shipped.
 ///
 /// Opt-in: which profile wins depends on why packets are lost — nc=1 deepens real congestion,
@@ -2461,7 +2459,7 @@ pub const OPTION_ENABLE_KCP_CC: &str = "enable-kcp-congestion-control";
 /// without a shaped link, so keep what users run today.
 #[inline]
 pub fn get_kcp_cc_enabled() -> bool {
-    Config::get_option(OPTION_ENABLE_KCP_CC) == "Y"
+    Config::get_option(keys::OPTION_ENABLE_KCP_CC) == "Y"
 }
 
 // this crate https://github.com/yoshd/stun-client supports nat type