test(host): make two host tests portable to Windows

The Windows host test suite hit two pre-existing portability failures
(the autonomous Windows bring-up never ran `cargo test` on the VM):

- `vdisplay::detect_active_session_*` asserted a non-empty XDG runtime
  dir — a Linux concept with no Windows equivalent. Gate just that
  assertion to Linux (keep the call so the fn stays used → no dead_code).
- `mgmt::openapi_document_is_complete_and_checked_in` did a byte compare
  against the checked-in spec, which git may check out CRLF on Windows
  while serde_json emits LF. Compare content with `\r` stripped.

Host suite now 73/73 on x86_64-pc-windows-msvc; Linux unchanged (78 ok).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay.rs

@@ -638,6 +638,8 @@ mod tests {
         // A pure probe of /proc + the runtime dir: it must not panic and must return promptly on
         // any box (CI has no graphical session → ActiveKind::None, with the runtime-dir anchor).
         let a = detect_active_session();
+        // The runtime-dir anchor is a Linux (XDG) concept; Windows has no equivalent.
+        #[cfg(target_os = "linux")]
         assert!(!a.env.xdg_runtime_dir.is_empty());
         // Wayland sockets are only resolved for the Wayland-protocol desktops.
         if matches!(