chore(clippy): appease 1.97 lints — derive Default, collapse ifs, drop needless borrow

Pre-existing, surfaced by the toolchain bump (rust-version is 1.95; the
sandbox gates with stable 1.97). All four are cargo clippy --fix output
with the mechanical-collapse indentation hand-tidied to house style;
the parser change is semantics-preserving (a non-100-continue Expect
value now falls to the _ arm instead of an empty if — headers.append
still runs after the match either way). No fmt pass: rustfmt would
clobber the aligned-assignment style, so only the touched lines moved.

examples/ws_chat.rs

@@ -92,7 +92,7 @@ impl WsHandler for ChatHandler {
         // subscribed as.
         let _ = self
             .bus()
-            .broadcast_from(smarm::self_pid(), &self.topic(), text);
+            .broadcast_from(smarm::self_pid(), self.topic(), text);
     }
 
     fn on_close(&mut self, _code: Option<u16>, _reason: &str) {