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/src-files.js

@@ -0,0 +1,2 @@
+createSrcSidebar('[["bookspec_checker",["",[],["derive.rs","engine.rs","files.rs","lib.rs","rules.rs","schema.rs"]]],["bookspec_gnucash",["",[],["lib.rs"]]],["bookspec_runner",["",[],["lib.rs"]]]]');
+//{"start":19,"fragment_lengths":[97,40,39]}