fix(network): restore bounded WebSocket frame transport

[seonghobae]

Current dependency and scope

Draft frame-transport repair at exact head 97fab641ed9d76e6c515eadcef0629edfc8064a3, based on opening-response owner #242 2d0e9f69df9ade21d8e8e3d807c3ff644d83b310. Two ordinary parent merges preserve existing history and the bounded frame-transport delta. No force push or destructive restack occurred.

The retained implementation provides masked UTF-8 text/Pong writes, adjacent masking-key reuse rejection, bounded unmasked server-frame reads, strict frame lengths/opcodes/control/Close validation, and typed fail-closed I/O/cleanup errors on the already verified transport. It neither reconnects nor grants browser, policy, secret or Agent authority.

Root-cause repair and integration

The descendant #255 complete Rust run exposed an invalid-deadline fixture that was already fixed at #242 but missing below the stale #243 edge. Adopting that parent exposed a second premature-close race in the mismatched-accept fixture. Owner #242 fixed the remaining exchange fixture paths with one shared test-only server that reads the full request and retains its stream through client assertions.

This PR adopts those canonical owner repairs. The fixture file is byte-identical to current #242; the unique parent-relative delta remains frame transport, public exports, masking regressions, integration doctoring and changelog. Production cleanup errors and security gates are unchanged.

Exact-head GREEN — 2026-09-08

Native CI 33952588687 is terminal success on unchanged exact 97fab641ed9d76e6c515eadcef0629edfc8064a3. Production coverage job 101270020891 passed measurement, diagnostics upload and exact production coverage enforcement. Rust contracts job 101270020934 passed Python repository contracts, canonical formatting, workspace check, full tests, strict Clippy and warnings-denied API documentation. The prior queued description is stale and is superseded by this exact-head evidence.

The earlier exact-tree local verification remains supporting evidence only: 139 Python contracts, Rust 1.97.1 fmt/check/tests/Clippy/rustdoc, and pinned-nightly production coverage at 884/884 functions, 8733/8733 lines, 11176/11176 regions and 1008/1008 branches. Hosted exact-head GREEN is now the controlling repository-quality evidence.

Remaining gates

Keep Draft while prerequisite #242 remains Draft. #242 itself still records an exercised dependency-root workflow-generation RED on repaired #195, owned by #279; this #243 GREEN must not bypass that prerequisite. Later descendants must retain parent-first adoption and regenerate their own exact-head evidence after foundation movement.

No counted approval, protected-main integration, browser-runtime acceptance, release, force update, destructive rebase, workflow/ruleset/secret mutation or self-approval is claimed.

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