test(store): the full-disk fault tests are Unix-only (ulimit -f, SIGXFSZ)

P11c run 11 on Windows: `bash` there is WSL's, with no distribution, and
the cap never bit. The simulation is Unix's; the books' full-disk
behavior is the same code everywhere.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-store/tests/faults.rs

@@ -7,6 +7,11 @@
 //! write fails; the parent then checks the books.
 //!
 //! Its own file: the child mode needs a test binary of its own to re-run.
+//!
+//! Unix only: the cap is `ulimit -f` under a Unix shell and the signal is
+//! SIGXFSZ; Windows has neither (its `bash` is WSL's, when one exists).
+//! The books' own full-disk behavior is the same code on every OS.
+#![cfg(unix)]
 
 use chrono::NaiveDate;
 use ledger_core::{AccountType, Currency, Money, Side};