feat(network)!: proper Zakura service discovery and peer-session refactor

[evan-forbes]

Motivation

Stacked PR 4/5 of the Zakura networking layer. Builds on PR3. Replaces the preliminary discovery (PR2) with proper service discovery and refactors peer sessions.

Solution

• Proper Zakura service discovery replacing the preliminary exchange.
• Peer-session refactor.
• Threads the discovery service through the header-sync registry tests.

Note: contains breaking changes to internal Zakura networking APIs (feat(network)!).

Tests

• Discovery + peer-session unit tests.
• cargo clippy --workspace --all-targets and cargo fmt --all -- --check clean locally.

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Superseded by a 3-PR restructure: only commits at clean boundaries (C1/C5/C8) compile + pass clippy standalone, so the 5-way split was collapsed into #106 (P2P stack),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).