docs: draft-v5 — mark C6, M8 and 11.5.8 closed

Phase 11.5 is complete. All six critical and all seven high findings from the
second review are now closed, bounded, or deferred by explicit decision.

Updated in place rather than appended, so the document does not carry stale
"open" markers next to shipped work:

  - §9 split into "closed since this document was drafted" and "still open",
    with C6, 11.5.6, 11.5.8 and M8 moved across and the closing mechanism
    recorded for each
  - §2 claim table: node impersonation is no longer pending
  - §3.1 QUIC now shows the unified handshake enforced
  - §4.2 records what the QUIC binding actually turned out to be, including the
    finding that a resumed TLS session carries no certificate, so the anchor
    travels with the session ticket
  - §4.4 states that the client pins pk_node and refuses a change

Added a scope note: with C6 closed, pinning is defence in depth, not the primary
control. A substituted node already fails the GEK proof; pinning covers the case
where an attacker holds the group key and swaps the node underneath.

H3 remains the last unfixed finding, and the document still says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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