fix: track fixture file in git and return proper errors

- Add negation rule in .gitignore so src/example_db/fixtures/*.json is
  tracked (fixes CI build failure: include_str! could not find the file)
- Replace expect() with proper ProblemError::SerializationError in
  build_example_db() fixture parsing
- Cache computed rule DB in tests via OnceLock to avoid redundant work

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: GiggleLiu

.gitignore

@@ -84,6 +84,7 @@ claude-output.log
 .worktrees/
 .worktree/
 *.json
+!src/example_db/fixtures/*.json
 .claude/worktrees/
 docs/test-reports/
 docs/superpowers/