virtio/console: backport control validation to governed v1.19.4

[dills122]

Summary

Backport the two separately retained virtio-console control-message fixes from the fork's upstream-oriented main line to a fresh governed v1.19.4 candidate.

• reject unknown PORT_READY and PORT_OPEN IDs before port or derived queue access;
• read exact eight-byte control payloads through the descriptor-chain Reader, accepting valid scatter/gather and rejecting malformed, writable, unreadable, short, or trailing data;
• complete every popped control transmit chain exactly once with used length zero;
• preserve the governed line's duplicate-start and shutdown/output behavior.

The source fixes are fork main commits eef8449c7438636139ff7fce553816e49ddc25d7 and 69e4fa9af5b9f37ffa23ba7bb87929a73ee0daf8 from #3. The backport commits are separate because v1.19.4 predates the current console refactor.

Governance

This PR also repairs the fork branch model exposed by the earlier merge:

• preserve original governed merge 4ea8d1de861ed1c0636fc800b6da8fb71a086aa5 under locked capsule/baseline-v1.19.4-r1;
• leave historical governed head cf0333cdba478cc34a8570a65b38412da7fd3ecc locked;
• target this PR to fresh candidate capsule/upstream-v1.19.4-r3;
• route governed CI by versioned branch prefix and retain the upstream integration/guest-test exclusion;
• update restoration mutations and exact test counts for the new queue-path tests.

Fork main remains upstream-oriented integration state. A merge there does not adopt bytes into Capsule; future adoption requires a separate governed backport.

While @dills122 is the only qualified maintainer, the mutable r3 target requires a pull request, green checks, resolved conversations, administrator enforcement, no force-push, no deletion, evidence/settings readback, and maintainer self-review, but GitHub requires zero approving reviews and does not require most-recent-push or CODEOWNER approval. External approval enforcement may be enabled when a second qualified maintainer is available. This does not waive DCO acceptance, later independent product-admission review, or final upstream-submission authorization.

Validation

• cargo test -p krun-devices virtio::console::device::tests --locked -- --nocapture — 5 passed
• governance/capsule-v1.19.4/scripts/verify-governed.sh — PASS
  • 53 default console/device tests
  • 4 bounded console coverage tests
  • 55 blk tests
  • Clippy with the single retained deprecation allowance
  • 25 shutdown and 25 queued-backpressure repetitions
  • AddressSanitizer PASS on local macOS arm64
  • 5 raw-FD and 4 console restoration mutations caught
• /opt/homebrew/bin/python3.12 .github/scripts/check-ai-trailers.py cf0333c — PASS
• workflow YAML parse and PATCH_QUEUE.json parse — PASS
• fixed upstream ./run.sh test --test-case multiport-console --keep-all — OK - 1/1 passed on macOS 26.5.2 arm64/HVF; retained at /tmp/libkrun-tests.18jnmpKrYQ3T
• governance-only single-maintainer follow-up: full local governed verifier, AI-trailer check, and git show --check — PASS;
• exact-head governed pull-request run 31021879532 and push run 31021880674 — PASS; formatting, AI trailers, code quality, cross-compilation, and Linux x86_64 unit tests also pass;
• Linux arm64 unit-test job in run 31021879591 remains queued for runner capacity, leaving GitHub merge state UNSTABLE without a test failure.

The governed library gate itself executed no guest. The separate fixed guest regression is bounded functional evidence only; it does not admit a backend, runtime, profile, release, or product path.

Review holds

• Keep this PR draft pending maintainer self-review and an explicit human DCO decision.
• The three functional commits and the governance-only follow-up intentionally have no Signed-off-by trailer. A human must review the result and explicitly accept DCO responsibility before merge.
• After acceptance, lock the resulting r3 head and make r3 the fork default.