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-package/src/package.rs

@@ -410,8 +410,12 @@ pub fn build_package<B: PackageBooks + ?Sized>(
     let mut used: std::collections::HashSet<String> =
         files.iter().map(|(n, _)| n.to_lowercase()).collect();
     let mut unique = |name: String| -> String {
-        let (stem, ext) = match name.rsplit_once('.') {
-            Some((s, e)) if !s.ends_with('/') && !s.is_empty() => (s.to_string(), format!(".{e}")),
+        // The extension is the last segment's own: a folder with a dot
+        // ("a.b/notes") keeps its name, and a name that is all extension
+        // (".env") has none.
+        let cut = name.rfind('/').map_or(0, |i| i + 1);
+        let (stem, ext) = match name[cut..].rfind('.') {
+            Some(i) if i > 0 => (name[..cut + i].to_string(), name[cut + i..].to_string()),
             _ => (name.clone(), String::new()),
         };
         let mut n = 1;
@@ -424,7 +428,8 @@ pub fn build_package<B: PackageBooks + ?Sized>(
     };
     if req.include_receipts {
         let clean = |s: &str| -> String {
-            s.chars()
+            let c: String = s
+                .chars()
                 .map(|c| {
                     if c.is_alphanumeric() || ".-_ ".contains(c) {
                         c
@@ -432,7 +437,14 @@ pub fn build_package<B: PackageBooks + ?Sized>(
                         '_'
                     }
                 })
-                .collect()
+                .collect();
+            // A name of dots alone ("", ".", "..") would name no file, or
+            // climb out of its folder when the zip is unpacked.
+            if c.chars().all(|c| c == '.') {
+                "_".repeat(c.len().max(1))
+            } else {
+                c
+            }
         };
         let mut index = String::from("entry_id,date,description,amount,file,sha256\n");
         for a in books.all_attachments().map_err(|e| e.to_string())? {