[philipgreat]

Summary

• introduce the dependency-free TeaQL error catalog and candidate scanner
• validate the catalog's supported YAML subset before checking allocations
• reject malformed top-level structure, indentation, list entries, and scalars with line diagnostics
• add focused regression coverage for malformed YAML

The preserved local baseline already includes direct test execution support and comment/literal masking; Issues #52 and #53 will be audited against main after this PR lands.

Verification

python3 -m unittest tools.test_error_catalog -v
python3 tools/test_error_catalog.py
python3 tools/error_catalog.py validate errors/catalog.yml
python3 tools/error_catalog.py scan --output /tmp/teaql-error-candidates-51.yml
git diff --check

Fixes #51
Related to #52
Related to #53