Implemented FLOW-001 operator fixes.

- Typed arithmetic excludes booleans.
- Added empty-string splitting.
- Update assignments use typed arithmetic/merge semantics.
- Added focused operator tests.

Validation: scoped conformance 174 passed, 0 failed/errors; full tests 79 passed.

RESULT: SUCCESS

FILES CHANGED:
- jq_interpreter/evaluator.py
- tests/test_flow_001_operators.py

SUMMARY:
Arithmetic and structural operator behavior is implemented and verified.

BLOCKERS:
- None