Implemented TEXT-001 string builtins.

- Added ASCII case conversion and parser registration.
- Fixed empty-string splitting and strict string validation.
- Added focused regression tests.
- `pytest -q`: 156 passed.
- Scoped conformance: 30 passed, 0 failed, 0 errored, 1 skipped.

RESULT: SUCCESS

FILES CHANGED:
- jq_interpreter/parser.py
- jq_interpreter/evaluator.py
- tests/test_text_001_string_builtins.py

SUMMARY:
String manipulation builtins now satisfy the declared scoped conformance suite.

BLOCKERS:
- None