refactor: v0.8 Iteration B — cruft purge + ALPN bump to itsgoin/4

Zero-users ruling removes all wire-compat obligations. Net -1,443 LOC of Rust.

Protocol break:
- ALPN_V2 b"itsgoin/3" -> ALPN b"itsgoin/4" (renamed: the versioned name was
  what rotted). Old nodes are now refused at the QUIC handshake instead of
  half-interoperating with the v0.8 manifest/digest formats. Deploy gate from
  Iteration A is discharged.

Dead wire surface removed (MessageType 46 -> 40):
- DeleteRecord 0x51 (deletes ride ControlOp::DeletePost + InitialExchange)
- VisibilityUpdate 0x52 + its two dead senders (ControlOp::UpdateVisibility)
- SocialDisconnectNotice 0x71 (zero senders; checkin timeouts carry the signal)
- MeshPrefer 0xB3 + request_prefer + handler (preferred peers are gone)
- Legacy dual pull-matching half (have_post_ids) — behavior-identical
- PullSyncResponse.visibility_updates + counters (control posts cover it)
- serde(default) stripped from wire-only payloads (kept where
  skip_serializing_if makes it load-bearing; persisted-row defaults untouched)

Preferred-peer subsystem eliminated (design ruling: CDN file_holders replaced
the N+10 direct push/pull it served): slot tier, preferred_peers table (dropped),
preferred_tree semantics, rebalance Priority 0, find_relays_for preferred tiers,
7-day prune + 30-day watcher, dead FromStr impl. Slots are now Local/Wide only
(91 desktop / 12 mobile) pending Iteration C's single ~20-slot pool.

Other dead code: hostlist encoder + base64url helper, always-empty
downstream_addrs parameter chain, start_upnp_renewal_cycle no-op + 4 callers,
RELAY_TARGET_RATE_LIMIT, GetSecretSeed, stale comments swept.

Preserved deliberately: EDM scanner corpse (awaiting raw-UDP refactor),
PortScanHeartbeat, Iteration A startup migrations, session-relay opt-in gating.

Docs: design.html + tech.html synced (message counts, ALPN, purge status,
Rework asides flipped past-tense).

190 core tests pass; CLI + desktop build; A3 integration 9/9 (independently
re-run post-purge).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LGiPD2cF75mnvneSCjdDC5

crates/core/src/profile.rs

@@ -68,7 +68,6 @@ pub fn apply_profile_post_if_applicable(
         updated_at: content.timestamp_ms,
         anchors: vec![],
         recent_peers: vec![],
-        preferred_peers: vec![],
         public_visible: true,
         avatar_cid: content.avatar_cid,
     };