Client connection setup already fails when QUIC DATAGRAM is unavailable and
clamps the advertised datagram budget before sending hello. Keep that clamped
value on ClientSession and expose it through QuicDiagnostics so the Windows
client can report the negotiated budget without recomputing handshake details.
The value remains owned by client-core because it is derived from the live
quinn connection and the configured client budget during handshake. TAP and UI
code can sample the snapshot later alongside tunnel counters.
Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-core
- cargo clippy -p lanparty-client-core --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
Refs: PLAN.md
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