fix(dht): seed routing with peer DHT port not gossip port (#1575 GAP 2)

[MichaelTaylor3d]

Corrects #1574's wrong-port routing bug — the resource read leg (#836) node->node unblock. Version 0.56.1 (PATCH — fix, not feat). GAP 1 (whole-capsule reshare) deferred to #1576; GAP 3 accepted as already-covered.

The bug

connected_pool_peers()/PoolEvents report a peer's GOSSIP addr (9445). #1574's live routing feed + the bootstrap seed fed that straight into the DHT routing table, so every DHT dial hit the peer's GOSSIP listener and died with received corrupt message InvalidContentType — find_providers therefore routed to nobody and every read fell back to RPC/upstream ("zero dht/download trace on a read" in the #1062/#1572 e2e). #1574 was necessary but wired the wrong port.

The fix

dht_addr_from_gossip_addr() maps the gossip addr down to the DHT/peer-RPC addr (9444) via the fixed GOSSIP_TO_DHT_PORT_OFFSET, applied at all three seed sites: the snapshot seed + PoolEvent::PeerAdded in spawn_dht_routing_feed, and the bootstrap_peers_from_pool seed. Once routing seeds a DIALABLE DHT contact, the existing Tier 2 read path (content_serve.rs peer_serve_plaintext -> fetch_resource -> locate_providers -> dig.fetchRange -> merkle-verify -> serve) moves content node->node — the #836 resource read leg.

Blast radius (socraticode + read)

crate::dht::add_peer callers = only the routing feed + bootstrap (both in peer.rs); no external consumer depends on the changed addr. bootstrap_peers_from_pool stays pure. Read-path only, no custody/signing. LOW risk.

Verification

• Regression test dht_candidate_from_pool_addr_uses_dht_port_not_gossip_port (9445->9444, IPv4 + IPv6 + custom-port via the offset) — FAILS on the old build.
• 144 peer-module tests green; fmt + clippy clean.
• Version-increment gate: 0.56.0 -> 0.56.1.

Closes #1575 (GAP 2 scope; GAP 1 -> #1576, GAP 3 -> confirmed on the #1572 re-run).

[MichaelTaylor3d]

PASS (loop-reviewer, correctness gate).

Verified #1575 GAP 2 fix:

• Direction/magnitude correct: GOSSIP_TO_DHT_PORT_OFFSET = 9445-9444 = 1; dht_addr = gossip - offset → 9445→9444, matching the peer.rs listener split (DHT/peer-RPC on 9444, gossip on 9445).
• Applied at ALL THREE routing-seed sites: snapshot seed (peer.rs:1330), PoolEvent::PeerAdded (peer.rs:1352), bootstrap_peers_from_pool feed (peer.rs:1987). None missed. No other DHT-routing add_peer caller feeds an unmapped addr.
• Preserves IP (set_port only); handles IPv4 + IPv6 + custom base port (offset-relative, saturating_sub guards underflow).
• Regression test dht_candidate_from_pool_addr_uses_dht_port_not_gossip_port is non-vacuous — asserts 9444 (not 9445) across IPv4/IPv6/custom; fails on the old build that fed the raw gossip addr.
• v0.56.0→0.56.1 patch; Cargo.lock refreshed in the same commit; version-increment gate green.
• Gates green: Rustfmt, Clippy, Test+coverage, CodeQL, commitlint.

No custody/crypto/signing touched (read-path only); single correctness gate is the right tier. No blocking findings.

Non-gating observation (not blocking, no action needed on this PR): ports 9444/9445 are pre-existing local consts — whether they should migrate to dig-constants is a separate, out-of-scope concern for this patch.