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-mcp/src/server_tests.rs

@@ -1159,7 +1159,9 @@ fn logged(tag: &str) -> (BooksServer, TestDir) {
 }
 
 /// Books whose access log can't be written: its file is a FIFO no one
-/// reads, so every record fails (as on a full disk).
+/// reads, so every record fails (as on a full disk). Unix only: there is
+/// no FIFO in a file's place on Windows.
+#[cfg(unix)]
 fn unwritable(tag: &str) -> (BooksServer, TestDir) {
     let books = unit_dir(tag);
     let store = store(&books);
@@ -1430,7 +1432,9 @@ fn a_call_the_books_close_or_stop_under_is_refused_as_not_run() {
 /// access.rs INVARIANT 3: a record that can't be written stops the
 /// books, and the answer says so; a call or read refused as not run
 /// (closing) keeps its not-run code, so the bridge still sends it
-/// again; anything else is an error in words.
+/// again; anything else is an error in words. Unix only: its fixture is a
+/// FIFO (the refusal path itself is the same on every system).
+#[cfg(unix)]
 #[test]
 fn an_access_record_that_cant_be_written_stops_the_books_and_says_so() {
     rt().block_on(async {