feat(net): accept relay hostnames

PLAN.md describes the first client flow as entering a relay domain and room
code, but the client and gateway CLIs only accepted socket-address literals.
Add a small shared RelayEndpoint parser so bare hosts default to UDP/443 while
IP literals and explicit host:port values stay supported.

The runtime configs still store resolved SocketAddr values. That keeps the
Windows route-pinning path on a concrete relay IP before TAP activation while
avoiding duplicated endpoint grammar between client and gateway. The relay
listen config reuses the same default port constant so UDP/443 has one source.

README examples now use lanparty-relay.local and document the shared endpoint
syntax.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-net
- cargo test -p lanparty-client-win \
  accepts_relay_domain_with_default_port -- --nocapture
- cargo test -p lanparty-gateway \
  accepts_iface_alias_for_gateway_interface -- --nocapture
- cargo test -p lanparty-net -p lanparty-client-win -p lanparty-gateway
- cargo clippy -p lanparty-net -p lanparty-client-win -p lanparty-gateway \
  --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

Cargo.lock

@@ -474,6 +474,7 @@ dependencies = [
  "lanparty-client-route",
  "lanparty-client-tap",
  "lanparty-ctrl",
+ "lanparty-net",
  "lanparty-obs",
  "lanparty-proto",
  "tokio",
@@ -498,6 +499,7 @@ dependencies = [
  "bytes",
  "clap",
  "lanparty-ctrl",
+ "lanparty-net",
  "lanparty-obs",
  "lanparty-proto",
  "libc",
@@ -507,6 +509,13 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "lanparty-net"
+version = "0.1.0"
+dependencies = [
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "lanparty-obs"
 version = "0.1.0"
@@ -531,6 +540,7 @@ dependencies = [
  "bytes",
  "clap",
  "lanparty-ctrl",
+ "lanparty-net",
  "lanparty-obs",
  "lanparty-proto",
  "quinn",