test: address parser limit review feedback

[GeorgMelinius]

Addresses all three unresolved Cubic findings on firecrawl#148:

• document that selected safety caps degrade gracefully instead of always returning ResourceLimit
• build the 200,000-level PPT regression fixture in linear time with one allocation
• use the unstarred RTF mmath destination so the test exercises MathState::open_group

Verified with Rust 1.88:

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --locked (290 unit tests, robustness test, and 9 snapshot tests passed; 1 local-samples test ignored)