docs(two-node): availability without the arbiter

[schmidt-scaled]

Docs only. Adds section 7.14 Availability without the arbiter to docs/design/two-node-arbitration.md:

• With arbitration on, a survivor needs the arbiter's grant (jc_grant_solo); without a verdict it fences itself at the hold deadline (2.5 s) or on lease expiry while holding. No preferred node.
• Table of situations → behaviour: node fails with/without the arbiter, arbiter down with a healthy pair (no effect), partition (fence_both, then unfence + leadership restore, 7.13), partition without the arbiter, frozen node (abort on wake, ultra Test vm stress #211), client path loss (multipath only), no arbitration (no split-brain protection).
• Rationale (third vote; consistency over availability), the deployment requirement (control plane HA or in a third failure domain), and the options considered (external witness: future; preferred-node fallback: removed 2026-10-12; documentation: done here and in docs(edge): two-node failover needs the hub — arbiter availability requirement documentation#242).

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