…is the sole locator (#386)

`PeerRpcClient` + `find_holders` + `PeerAvailability` predated the DHT and had ZERO
live callers (only definitions + a stale doc claiming "#163 dig-dht plugs in behind
Self::find_holders" — an integration that never landed; the DHT went in via
`NodeContent` instead). Removing the dead seam leaves ONE unambiguous provider path:
the dig-dht provider index (`DhtProviderLocator` -> `find_providers`) inside
`NodeContent`, used by both the redirect-on-miss and multi-source fetch paths.

Rewrites the module doc's stale "clean seam for #163" note accordingly.

Refs #376, #253
Co-Authored-By: Claude <noreply@anthropic.com>

Learned peer addresses (PEX, dig.getPeers, introducer, observed pool peers) were
dialed-immediately-or-discarded — nothing accumulated a durable, ordered address
book to seed later candidate selection. Adds `address_book::AddressBook`: one entry
per peer_id, unions dialable addresses IPv6-first (§5.2), records provenance (a
first-hand sighting is not downgraded by a later PEX hint) + a freshen timestamp,
persists relay-only / not-currently-dialable hints, and reads back a ranked,
non-stale candidate list (IPv6-dialable first, then dialable, then relay-only) with
a staleness TTL + capacity eviction.

Crate-API note (release-first follow-up): dig-gossip owns the canonical
AddressManager but exposes NO public production ingest API (only a #[doc(hidden)]
test hook), so this book lives node-side for now; when dig-gossip ships a public
`offer_addresses` ingest API the book flushes into the crate AddressManager (one
source of truth). Documented in the module doc + SPEC §19.7.

Tests: IPv6-first ordering + dedup, relay-only persistence, upsert union/freshen,
provenance non-downgrade, readback ordering, TTL staleness, capacity eviction.

Refs #376
Co-Authored-By: Claude <noreply@anthropic.com>

…l ordering (#387, #384)

Two coupled changes to the PEX pool sink (`GossipPexPool::offer_candidates`):

#387 — feed PEX-discovered peers into the address book, not dial-and-drop. Every
learned candidate (INCLUDING relay-only / not-currently-dialable hints) is offered
into the durable address book (#381) with provenance PEX, so nothing is discarded;
a hint that can't be reached now persists to seed a later dial. Dials are now sourced
from the accumulated book (a superset of this batch), so previously-learned +
relay-only-turned-dialable hints are retried; a failed dial keeps the hint in the book.

#384 — let dig-peer-selector drive dial choice. Adds a `DialRanker` seam
(`SelectorDialRanker` over the shared PeerSelector via its content-agnostic
peer_snapshot) that orders which book candidates are dialed first — reliability
blended with throughput, banned peers last, cold peers explored at a neutral rank.
Reuses the ONE selector instance already ranking download sources (never a second);
IPv6-first order preserved among equally-ranked peers.

Crate-API note (release-first follow-up): dig-gossip's PeerPoolConfig exposes NO
dial-priority hook, so selector ordering applies to the node's PEX candidate dials;
when dig-gossip ships a pool dial-priority hook the same ranking drives the pool's
own maintenance dial loop. Documented in SPEC §19.6/§19.7.

Removes the now-unused `best_socket_addr` helper (superseded by the book's IPv6-first
CandidateAddr ordering). Tests: entry→candidate conversion (dialable IPs, relay-only),
ranker ordering (good→neutral→bad, None ranker no-op).

Refs #376, #381
Co-Authored-By: Claude <noreply@anthropic.com>

Adds SPEC §19 (peer network: full NAT ladder, STUN reflexive discovery, DHT-only
locator, durable address book, PEX handling, selector-driven dial ordering, and the
release-first crate-API integration status). Applies rustfmt across the P2P-wiring
changes and bumps the workspace to v0.19.0 (dig-node-core 0.6.0) — MINOR: new
node-side capability (address book + dial ordering), backwards-compatible.

Refs #376, #381, #384, #385, #386, #387
Co-Authored-By: Claude <noreply@anthropic.com>