Fix failed XHR accounting and tighten documented guarantees

[godhiraj-code]

Summary

• balance pendingRequests when synchronous XMLHttpRequest.send() throws before loadend
• add a real-browser regression test preserving the original exception and verifying the counter returns to zero
• replace overstated README/package claims with verified behavior
• prepare version 1.0.3 so the existing PyPI 1.0.2 release is not overwritten

Testing

• pytest -q: 163 passed, including Chrome integration tests
• wheel and sdist built as 1.0.3
• twine check: both artifacts passed

Risk

Low. The runtime change handles only the synchronous exception path and rethrows the original browser error after balancing internal accounting.