Report invalid selectors without panicking

[VrtxOmega]

Fixes #65.

Problem

Unsupported CSS selectors such as div:has(h1) make htmlq panic while parsing the selector. The CLI exits 101 and can emit a Rust backtrace instead of a user-facing error.

Change

Handle the selector parser error explicitly, print the exact invalid selector, and exit with status 2.

This does not add :has support or change successful selector behavior.

Verification

• Reproduced the original exit 101 panic on current master
• Added a regression test requiring exit 2 and no panic
• cargo test --all-targets — 10 passed, 0 failed
• Manual invalid-selector check — exit 2, exact diagnostic, no panic
• Manual valid div selector check — output unchanged

Risk

Low. The change affects only selector-parse failures and adds no dependency.