fmt: rustfmt breaks the tuple-index chain, and the desktop lane means it

`cargo fmt --all --check` failed the desktop lane on one hunk in the new
`client_headers_identify_app_and_protocol` test. Clippy and every test
passed; only the formatter objected.

rustfmt splits `client_headers()[0].1.starts_with(..)` across lines because
an index followed by a tuple field followed by a call is a three-element
chain, and it will not keep one on a single line inside a macro argument
regardless of width. This is rustfmt's own output, pasted back.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K3MMqUtzX1TJgA1oypvm1c

core/src/sync/compat.rs

@@ -411,7 +411,9 @@ mod tests {
         // this one for the OnceLock, and whichever lost would see the other's name.
         // One test, both branches, in order.
         assert!(
-            client_headers()[0].1.starts_with("thoughtsync-unidentified/"),
+            client_headers()[0]
+                .1
+                .starts_with("thoughtsync-unidentified/"),
             "a host that never introduced itself must say so"
         );