[cppla]

Summary

• Track CONNECT-UDP session workers independently of the target lookup index, so PacketConn Close also joins workers belonging to already-removed sessions.
• Register logical packet cancellation watchers under the client's existing closed-state lock, so WebH3Client Close waits for packet shutdown as well.
• Preserve admission release timing, target reopening, healthy UDP behavior, authentication, wire format and defaults. Synchronize architecture documentation.

Regression and local verification

• Deterministic owned TLS 1.3/H3 regression with real authenticated request/stream reset, plus a test-only scheduling barrier before worker cancellation selection. Both packet-close and client-close cases reproduce early return against only the original production source; all fixture workers independently join. The same final test passes with this fix.
• Go 1.25.13 focused race/shuffle count 10: seven top-level tests, 140 RUN/PASS nodes, no FAIL/SKIP/race markers. Existing healthy echo, late opening, capacity reuse, cancellation-cause isolation and physical cleanup controls included.
• Go 1.27.1 make check and make race: passed.
• Offline isolated Linux/arm64 Docker execution with fresh Go 1.25.13 CGO=0 binaries: four top-level tests x3, 27 RUN/PASS nodes. Driver/container/source/binary/cleanup guards passed; temporary container removed. This container run is non-race.

Scope and evidence limits

The scheduling barrier is synthetic, not a natural network failure-rate experiment. The regression uses the normal successful registration/start handoff tail after a real authenticated open, without a production hook. Second concurrent Close startup is observed, not its internal stack. Full Go gates have nonverbose package output.

No dependency or protocol/default change, remote deployment, release/tag/registry publication, real-browser comparison or corpus run. Exact-head PR and merged-main CI are independent publication gates.

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