feat(dht): add browser transport addresses and shared iterative lookup

[mickvandijke]

Summary

Adds the transport-independent address and lookup foundations needed by direct browser clients while keeping native DHT behavior on the same implementation.

This PR introduces canonical WebRTC Direct transport addresses, a versioned extensible address-publication plane, and a small runtime-independent Kademlia lookup crate shared by native Saorsa and browser/WASM clients.

What changes

Canonical transport addresses

• Adds WebRTC Direct multiaddresses carrying the literal IP, UDP port, DTLS certificate multihash, and expected peer ID.
• Retains the earlier WebTransport address representation as a generic transport variant.
• Centralizes parsing, validation, serialization, and reachability classification in Saorsa types rather than browser-specific JavaScript.

Versioned address publication

• Freezes the existing V1 PublishAddressSet wire representation as its QUIC-only projection.
• Adds a V2 transport-address record with numeric transport/reachability discriminants.
• Adds V2 publish/find-node handling and identity capability negotiation.
• Keeps one canonical in-memory address set while returning the appropriate V1 or V2 projection to each peer.
• Allows browser endpoints to propagate through the authenticated DHT without changing the legacy enum.

Shared iterative lookup

• Adds the saorsa-dht-lookup workspace crate with no Tokio or transport dependency.
• Extracts candidate ordering, alpha batching, failure state, bounded candidate retention, grace collection, and convergence into one generic engine.
• Refactors native DhtNetworkManager to use that engine.
• Makes the same lookup policy consumable from ant-core WASM instead of reimplementing Kademlia traversal in JavaScript.
• Documents the decision in ADR-016.

Compatibility

• V1 address publication remains byte-compatible and QUIC-only.
• V2 is capability-selected; older peers continue using V1.
• Existing native lookup still supplies transport authentication, consensus, trust updates, and failure-cache behavior around the shared policy engine.
• No stored-data format or payment changes.
• Depends on saorsa-transport#160, pinned by commit SHA for draft CI reproducibility.

Test evidence

• cargo check --workspace
• cargo test -p saorsa-dht-lookup
  • 10 tests passed.
  • Covers XOR ordering, alpha limits, bounded eviction, multi-round traversal, failure-state monotonicity, grace collection, straggler cancellation, and top-K convergence.
• cargo fmt --all -- --check
• git diff --check
• The shared runner is additionally exercised from generated ant-core WASM and the live Chromium/node integration test.

Risk and review focus

• The V2 address plane and native lookup refactor affect DHT behavior and deserve protocol/routing review.
• Review should focus on V1 wire preservation, capability negotiation, convergence equivalence, and failure-state handling.
• Linear issue: TBD before the stack is marked ready.
• This branch started from saorsa-core v0.26.4 and currently needs rebasing onto latest main.

Rollback

Revert V2 publication and the native lookup adapter. V1 QUIC address publication remains the compatibility baseline.

Coordinated draft stack

• Transport: saorsa-transport#160
• This address/lookup layer: saorsa-core#158
• Portable protocol: ant-protocol#29
• Node/browser RPC: ant-node#220
• ant-core/WASM client: ant-client#186