chore(lint): clear all deny-level clippy findings across the workspace

[bobleer]

Why

[workspace.lints.clippy] sets correctness and suspicious to deny, but several crates fail to compile under cargo clippy --workspace --all-targets. Clippy short-circuits on the first compile error, so it never reached those crates — which had accumulated a backlog of hard errors that a green --all-targets run would reject.

cargo clippy --workspace --all-targets now reports 0 errors.

Compile blockers

• relay-server/tests/library_compat.rs: the AppState initializer had fallen behind the struct (missing page_browser_auth), and let _ = routes::api::server_info(); built an unpolled future instead of pinning the symbol.
• rmcp API drift: SseStream::from_byte_stream → from_bytes_stream.

Deny-level fixes

• await_holding_lock (25 sites). The subscription-auth and account-context test serialization locks now use tokio::sync::Mutex, matching the store_lock precedent already in the same file; the plain #[test] cases take them with blocking_lock, which cannot stall without an ambient runtime. The remaining sites scope their guard in a block. Note that an explicit drop(guard) before the await does not satisfy this lint — it reasons over lexical scope.
• suspicious_open_options. The json-store lock file now states truncate(false): it carries no payload and must never be truncated while another process holds the flock on that inode.
• items_after_test_module, never_loop, octal_escapes, unnecessary_get_then_check, drop_non_drop, plus the mechanical style/complexity/perf fixes from clippy --fix in the crates that were previously unreachable.

Two judgment calls worth a look

• client_info.rs keeps advertising the SEP-2577-deprecated roots/sampling capabilities behind a scoped #[allow(deprecated)] with the rationale recorded inline. Dropping them is a protocol-visible behavior change pinned by mcp_remote_client_info_declares_supported_client_capabilities, not a lint cleanup.
• stat_local_path_metadata keeps its expanded symlink check behind #[allow(clippy::needless_bool)]. Clippy only sees the #[cfg(not(windows))] { false } arm and would otherwise collapse away the Windows reparse-point detection — a silent platform regression.

Scope

Unsafe blocks in the SSH plugin source, terminal exec, and webdriver capture/evaluator paths are now documented. No behavior change anywhere else.

Left alone deliberately, since they need design judgment rather than a mechanical fix: too_many_arguments, large_enum_variant, type_complexity, and the ~195 undocumented unsafe blocks in the macOS accessibility FFI under desktop/src/computer_use/macos_*.