stream: track FIN acknowledgements separately

[xgame92]

Summary

• track FIN acknowledgement separately from acknowledged stream byte ranges
• retain streams until a FIN-bearing STREAM frame is acknowledged
• preserve reset and shutdown completion behavior
• cover zero-length FIN delivery and retransmission with Cubic and BBR2

Testing

• cargo +nightly fmt -- --check
• cargo clippy --release -p quiche --all-targets -- -D warnings
• cargo test --release --all-targets --features=async,ffi,qlog --workspace
• cargo test --release --doc --features=async,ffi,qlog --workspace

Fixes #2525