fix(ci): format lib/borrow_polonius/dune to satisfy dune fmt

#614 added lib/borrow_polonius/dune unformatted; the build job's
`dune build @fmt` requires a blank line between the comment block and
the (library ...) stanza. It was masked on main until #617 fixed the
doc-truthing guard that failed earlier in the same job.

Only the blank line is added (canonical `dune format-dune-file` output);
no other dune files are touched — root dune-project / .build/dune-project
"drift" only under newer local dune and is not flagged by CI's dune.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lz7pRcec2Z3tVtaAhvB3M8

Author: hyperpolymath

lib/borrow_polonius/dune

@@ -1,6 +1,7 @@
 ; ADR-022 M1: the Polonius-style loan solver lives in its own library, created
 ; but NOT yet wired into `affinescript` (nothing depends on it). M3 wires it in
 ; behind a parallel-run diff against the lexical checker; M4 cuts over.
+
 (library
  (name borrow_polonius)
  (public_name affinescript.borrow_polonius)