chore: raise rust-version floor to 1.88; fix 1.88 clippy lints

The 1.85 claim was already false at the dependency level: the resolved
lockfile pulls icu_* 2.x (MSRV 1.86) and wasip2 1.87 transitively via
jsonschema -> idna, so 1.85 toolchains cannot build 0.7.0 regardless of
what Cargo.toml declares. Raising the floor to 1.88 aligns with the
noq QUIC path (per the ecosystem MSRV audit) and breaks no downstream
that could build the crate before.

- Cargo.toml: rust-version 1.85 -> 1.88 (version-line bump deferred to
  release time; semver-checks passes either way)
- src/channels/mux.rs: collapse else { if .. } (collapsible_else_if)
- src/core/types.rs, src/registry/registration.rs: inline format args
  (uninlined_format_args)

Verification:
- 1.88: cargo test --locked (631 pass), clippy --all-targets -D warnings
- stable 1.94: test --locked, clippy -D warnings, --all-features, fmt,
  doc --no-deps
- wasm32-unknown-unknown: cargo check
- cargo semver-checks check-release: no semver update required

src/channels/mux.rs

@@ -151,18 +151,15 @@ impl MuxRunner {
                                     );
                                 }
                                 break;
-                            } else {
-                                if let Err(e) =
-                                    super::wire::write_chunk(&mut *writer, channel_id, &payload)
-                                        .await
-                                {
-                                    tracing::warn!(
-                                        channel_id,
-                                        error = %e,
-                                        "mux pump: failed to write chunk"
-                                    );
-                                    break;
-                                }
+                            } else if let Err(e) =
+                                super::wire::write_chunk(&mut *writer, channel_id, &payload).await
+                            {
+                                tracing::warn!(
+                                    channel_id,
+                                    error = %e,
+                                    "mux pump: failed to write chunk"
+                                );
+                                break;
                             }
                         }
                         None => {