Reject unknown P2P message codes as malicious

[a-shannon]

Classify unregistered P2P message codes as malicious protocol input so the existing permanent-penalty, blacklist and connection-teardown path handles them. Previously MessageSerializer returned Failure(Error); PeerConnectionHandler logged that generic failure, resumed reading and retained the invalid prefix, parsing it again as later data accumulated. Returning MaliciousBehaviorException selects the existing penalty/teardown behavior. Known codes, incomplete frames, payload bounds and the wire format are unchanged.

Head b2e0c4690a836f124ba7bf831887a32140873776 remains targeted at v6.0.7, base e927ab38e652423adcb79a41e8a052e348545320. The original two-file correction, including its complete zero-payload unknown-code regression, is unchanged.

The new eleven-file prerequisite increment adds settled-seed/isolation D, owned by #2535, plus G, owned by #2511, and X, owned by #2549. G corrects full-summary anchors, tip/mode cache identity and retained-common-ancestor download scheduling. X revisits cached sections after their headers arrive. This adds four shared production paths and their tests to the cumulative PR; it does not alter the message-code correction.

Review the exact shared reader prerequisite R, owned by #2535, before G, then merge the owning #2511/#2549 corrections before this consumer. Existing H, U and T remain unchanged. Selected-header targets, all convergence assertions and original deadlines are retained. T's separate 2.5-second failure-only collection still rethrows the original timeout. Exact common commits avoid importing unrelated owner branches.

At 6fe05788, native IT passed 49/50 with one ignored case; DeepRollBack alone timed out at minimum 221. The older IT job passed 12/13 with one ignored case and failed the flattened stored-header comparison. These observations are historical; G/X are independently demonstrated corrections, not exclusive attribution of either CI failure.

Related scopes remain distinct: #2320 substitutes ErgoException, which still takes the log-only path; #2366 checks peer versions after message identification; declared-length bounds are upstream via #2306, covering closed #2381; and #2404/#2405 reject trailing payload bytes.

The actual final composition passed normal compilation and 49/49 focused synchronizer/history/cache tests, including UTXO and Digest wake-up cases. Its preceding D composition passed normal IT compilation and H/D 33/33. Unchanged prior H/T 37/37, MessageSerializer 5/5 and network 71/71 evidence remains historical and input-qualified; the original negative control failed with java.lang.Error before the correction. Independent source review found no blockers. Local checks did not run Docker or establish full CI success. New-head native CI remains pending in #2533.