test(scanner): add browser submit fixture

[adity982]

Summary

• add a synthetic, local-only browser form submission fixture
• assert the universal scanner recognizes the browser side effect as agent-controlled runtime behavior
• guard against definitive vulnerability or severity claims for the fixture

Why

Issue #4 asks for a realistic browser-submit sample that expands scanner coverage without introducing network access or sensitive data.

Validation

• uv run pytest tests/unit/test_scanner_universal.py -q — 27 tests and 15 subtests passed
• uv run ruff check examples/scanner_browser_submit/browser_submit.py tests/unit/test_scanner_universal.py — passed
• badge sync check — passed
• README install checker — passed with UTF-8 mode enabled

The broader scanner/CLI selection passed 33 tests; two existing CLI cleanup tests failed on Windows because SQLite retained a file lock. The complete suite exceeded a five-minute local limit, and the repository's Make targets are unavailable in this Windows environment.

Closes #4