docs(v0.17.0): robust WAN join + presence-liveness truth — design checkpoint

Off two confirmed field incidents this session (SCELTOUIN): (1) WAN join silently
failed on half-broken IPv6 — iroh dual-stack burned the rendezvous window on dead
AAAA candidates; (2) a remote node painted another node's DEAD endpoints ONLINE
(dead perch gossiped Dormant @registryhost.rs:404, picker maps remote Dormant→Online).

- ADR-0030 (robust WAN subnet join): two-phase meet-before-code + per-family bind gate
  + diagnosable join. Grilled with the operator (/grill-with-docs).
- docs/design/robust-wan-subnet-join.md: per-item spec.
- CONTEXT.md: corrected the TOTP-epoch (public, routes the meet) vs TOTP-code (secret,
  SPAKE2 only) conflation — the keystone that licensed meet-before-code.
- traceable-reqs.toml: REQ-JOIN-TWO-PHASE / REQ-NET-FAMILY-GATE / REQ-JOIN-DIAGNOSTICS
  (doc-ratified by ADR-0030, required_stages=[doc]) + REQ-PRESENCE-LIVENESS-TRUTH (seed []).
- V0.17.0-ROBUST-JOIN-AND-PRESENCE-JIT.md: 5-wave build path (W1 family-gate + W4
  presence-truth = the correctness blockers; W2 two-phase + W3 diagnostics = UX hardening).

Co-authored by: doyle