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.

src/conn.rs

@@ -158,7 +158,9 @@ impl Body {
 // Empty `Vec`s are skipped by the pump (a zero-length chunk would
 // terminate chunked framing early), so they are safe to send but useless.
 
+#[derive(Default)]
 pub enum RespBody {
+    #[default]
     Empty,
     Bytes(Vec<u8>),
     Stream(StreamBody),
@@ -217,10 +219,6 @@ impl RespBody {
     }
 }
 
-impl Default for RespBody {
-    fn default() -> Self { RespBody::Empty }
-}
-
 // ---------------------------------------------------------------------------
 // Params — path parameters extracted by the router.
 // ---------------------------------------------------------------------------