Windows: link the overlay's mirrored files instead of copying them

`link` copied every file the mirror hands the overlay, so a selection with a
pack on wrote the prepared root out again: 639 MB and 28 181 files here, and a
cold overlay build of 34.4 s against 20.8 s when the same files are hardlinks.
Unix already links them and the overlay only reads them.

A different volume, or a filesystem without hardlinks, still copies.

Co-Authored-By: Hermes Agent (Nous Research) <[email protected]>

crates/horizon-client-render/src/packs.rs

@@ -380,7 +380,11 @@ fn link(target: &Path, at: &Path) -> io::Result<()> {
     if target.is_dir() {
         Ok(())
     } else {
-        fs::copy(target, at).map(|_| ())
+        // The mirror only reads these files, and unix links them: a hardlink gives
+        // the same view without writing the 639 MB prepared root again, which was
+        // 33 s of a cold Windows launch with a pack on. Another volume (or a
+        // filesystem without hardlinks) still copies.
+        fs::hard_link(target, at).or_else(|_| fs::copy(target, at).map(|_| ()))
     }
 }
 /// Build `out` as `vanilla` with the packs' files layered on top (packs in