fix: git gets plain Windows paths; the FIFO test is Unix-only (P11c)

std::fs::canonicalize on Windows returns \\?\C:\..., which git refuses
("not a git repository"): every argument handed to git now has the
extended-length prefix taken off (root::without_extended_prefix, with a
unit test that runs everywhere; \\?\UNC\ becomes \\). The access-record
test's fixture is a FIFO in the log's place, which Windows has no
counterpart for; the refusal path is the same code on every system, so
the test and its helper are Unix-only. main (8f0e5f08) merged in.

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

crates/ledger-store/src/backup.rs

@@ -223,7 +223,13 @@ impl Git {
     fn run(&self, what: &str, args: &[&std::ffi::OsStr]) -> Result<String, BackupError> {
         use std::sync::atomic::{AtomicU64, Ordering};
         let mut c = self.command();
-        c.args(args);
+        // A path made canonical on Windows carries `\\?\`, which git refuses.
+        c.args(args.iter().map(|a| match a.to_str() {
+            Some(text) if text.contains(r"\\?\") => {
+                std::ffi::OsString::from(crate::root::without_extended_prefix(text))
+            }
+            _ => (*a).to_os_string(),
+        }));
         #[cfg(unix)]
         {
             use std::os::unix::process::CommandExt;