Add acked packet count to connection stats to make short-term loss rate more precisely

[xxoo]

Why

Using lost / sent for short-term loss estimation is misleading because sent includes a significant share of packets that are still in flight.
For immediate control decisions, this inflates the denominator and underestimates actual loss.
This change uses resolved outcomes instead: lost / (acked + lost) or (lost - spurious_lost) / (acked + lost).

What changed

• Added acked (number of acknowledged packets) to quiche::Stats.
• Extended ACK processing outcomes with acked_packets and propagated it to connection-level stats.
• Added acked to C FFI quiche_stats and populated it in quiche_conn_stats().
• Updated apps/quiche-server loss-rate calculation from lost / sent to lost / (acked + lost), with zero-denominator protection.
• Added/updated tests, including a dedicated test that validates stats().acked.

Compatibility

• Rust API breaking: quiche::Stats has a new public field (acked).
• C ABI breaking: quiche_stats layout now includes acked.

Validation

• cargo test --all-targets --features=async,ffi,qlog --workspace
• cargo test --doc --features=async,ffi,qlog --workspace

[chatgpt-codex-connector]

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