fix(linter): avoid VAR13 shadowing false positives

[bhirsz]

Summary

• match automatic variables by exact identifier and normalized name, excluding environment variables and unsupported list forms
• suppress VAR13 when reserved names are statically shadowed by Variables-section bindings, assignments, VAR, or BuiltIn Set ... Variable calls
• model setup/teardown runtime ordering conservatively and prevent local bindings from leaking between tests
• add RF 5/6/7 regressions for ordering, scope, item/typed assignments, nested ranges, setters, and delayed teardown contexts

Validation

• uv run pytest -q tests\linter\rules\variables (55 passed, 15 skipped)
• exact Robot Framework 5.0.1, 6.1.1, 7.0.1, and 7.3.2 VAR13 runs
• uv run ruff check
• uv run ruff format --check
• uv run mypy --config-file=pyproject.toml .\src\robocop\
• independent review, with setup and suite-setup ordering findings addressed

Follow-up to #1867.