P2: tests mutation found, three bugs fixed; plan P12 and P13

Mutation testing pauses until P12 (D-33, owner 2026-10-20), which reruns
the whole max-testing pass over everything since P1. Before the pause,
pass 1 on slices 3-6 and its triage gave tests in ledger-store,
ledger-package and ledger-mcp, and three real bugs, fixed:
- the command line took an admin answer longer than four MiB, or cut off
  before its newline, whenever the bytes that came still parsed;
- The CPA package put a twin's number on the last dot of the whole path,
  so in a folder with a dot the second file landed in another folder;
- a document or category named of dots alone ("..") could climb out of
  its folder when the CPA package is unpacked.
The plan gains P13, bookspec (D-34): a formal, sourced accounting
specification first (rules with symbolic forms and worked examples), then
its own formal validation and a conformance harness any engine can be
tested against, seeded by P3's regression tests. docs/testing.md records
slices 3-8 and what P12 must cover; plan/STATUS.md says where P2 stands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-mcp/tests/spec_kept.rs

@@ -221,6 +221,13 @@ fn a_settings_file_moves_into_the_books() {
         json!({"changes": [{"key": "company.cpa", "value": "Casey"}], "decided_by": "A"}),
     );
     assert!(refused.contains("books.import_settings"), "{refused}");
+    // Nor is there an export yet: the file is the copy.
+    let refused = m.call_err("books.export_settings", json!({}));
+    assert!(
+        refused.contains("still read their settings file")
+            && refused.contains("books.import_settings"),
+        "{refused}"
+    );
     // The preview records nothing.
     let head = |m: &mut Mcp| m.call("books.start", json!({}))["head"]["seq"].clone();
     let before = head(&mut m);