feat(checker): examples() and spec_files() carry the specification's files

A consumer of the checker crate needs no path into this repository.

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

target/doc/trait.impl/bookspec_checker/engine/trait.Engine.js

@@ -0,0 +1,9 @@
+(function() {
+    const implementors = Object.fromEntries([["bookspec_gnucash",[["impl <a class=\"trait\" href=\"bookspec_checker/engine/trait.Engine.html\" title=\"trait bookspec_checker::engine::Engine\">Engine</a> for <a class=\"struct\" href=\"bookspec_gnucash/struct.GnuCash.html\" title=\"struct bookspec_gnucash::GnuCash\">GnuCash</a>",0]]]]);
+    if (window.register_implementors) {
+        window.register_implementors(implementors);
+    } else {
+        window.pending_implementors = implementors;
+    }
+})()
+//{"start":59,"fragment_lengths":[288]}