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

checker/Cargo.toml

@@ -7,6 +7,7 @@ repository.workspace = true
 description = "bookspec's example schema, its rule identifiers and the Engine trait an accounting engine implements."
 
 [dependencies]
+include_dir = "0.7"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "1"