feat(dht): Phase 2 engine — real iroh-blobs provider behind iroh-swarm

Pulls iroh 1.0 + iroh-blobs 0.103 as OPTIONAL deps under the iroh-swarm
feature and implements a real BlobProvider over them. Verified: the full
iroh QUIC dep tree (260 pkgs) resolves and compiles against the pinned
bitcoin/nostr-sdk/reqwest-rustls stack; the provider compiles against the
0.103/1.0 API.

- swarm/iroh_provider.rs: IrohProvider::new binds a QUIC Endpoint, opens a
  persistent FsStore (data_dir/iroh-blobs), and serves blobs via the
  iroh-blobs protocol/Router — a node that fetches also SEEDS. try_fetch
  maps ContentDigest -> iroh Hash, asks discovery for seed EndpointIds, then
  downloader.download(hash, providers) (range-verified) + export to staging.
- ProviderDiscovery trait: the seam Phase 3 (signed Nostr advertisement
  events) fills. discovery=None -> no seeds -> origin-only, so enabling the
  feature is never worse than today.
- Default build untouched: iroh is optional, the module is cfg-gated, and
  providers() stays empty until Phase 3 wires discovery in.

Build: cargo build --features iroh-swarm succeeds (dev). Default build +
44 swarm/update/content_hash/blobs tests unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/archipelago/Cargo.toml

@@ -17,7 +17,7 @@ default = []
 # is empty and every fetch goes straight to the origin HTTP path (today's
 # behaviour). Attach the optional iroh / iroh-blobs deps to this feature when
 # wiring the IrohProvider.
-iroh-swarm = []
+iroh-swarm = ["dep:iroh", "dep:iroh-blobs"]
 
 [dependencies]
 # Core dependencies
@@ -117,6 +117,12 @@ sd-notify = "0.4"
 # Trait objects for async methods (container orchestrator trait, Step 4)
 async-trait = "0.1"
 
+# DHT Phase 2: iroh-blobs peer swarm engine. OPTIONAL — only pulled in by the
+# `iroh-swarm` feature (off by default). Heavy QUIC dep tree; kept behind the
+# flag so the default fleet build is unaffected until the PoC is measured.
+iroh = { version = "1", optional = true }
+iroh-blobs = { version = "0.103", optional = true }
+
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3.10"