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/core/str/traits/trait.FromStr.js

@@ -0,0 +1,9 @@
+(function() {
+    const implementors = Object.fromEntries([["bookspec_checker",[["impl <a class=\"trait\" href=\"https://doc.rust-lang.org/1.99.1/core/str/traits/trait.FromStr.html\" title=\"trait core::str::traits::FromStr\">FromStr</a> for <a class=\"struct\" href=\"bookspec_checker/engine/struct.RuleId.html\" title=\"struct bookspec_checker::engine::RuleId\">RuleId</a>",0]]]]);
+    if (window.register_implementors) {
+        window.register_implementors(implementors);
+    } else {
+        window.pending_implementors = implementors;
+    }
+})()
+//{"start":59,"fragment_lengths":[321]}