feat(network): add Zakura native peer and service discovery

[evan-forbes]

Motivation

Stacked PR 2/5 of the Zakura networking layer. Builds on #PR1 (P2P stack + legacy adapter). Adds the preliminary native peer and service discovery exchange so Zakura nodes can find each other and advertise services natively.

Solution

• Native peer- and service-discovery exchange over the Zakura transport.
• Reorganizes the Zakura transport into Service / Sink / Source packages and ports native discovery onto it.

Tests

• Zakura discovery testkit + unit tests.
• cargo clippy --workspace --all-targets and cargo fmt --all -- --check clean locally.

Follow-up Work

• The preliminary discovery here is superseded by "proper service discovery" in stacked PR 4/5.

AI Disclosure

• AI tools were used: Claude Code (Opus) for rebase/conflict resolution, commit organization, and this description.

PR Checklist

• Conventional-commit title
• Solution is tested

[evan-forbes]

Superseded by a 3-PR restructure: only commits at clean boundaries (C1/C5/C8) compile + pass clippy standalone, so the 5-way split was collapsed into #106 (P2P stack), #111 (discovery + header sync + block sync), and #112 (block-sync hardening/refactor/stabilization).