feat(network): add Zakura native header-sync service

[evan-forbes]

Motivation

Stacked PR 3/5 of the Zakura networking layer. Builds on PR2. Adds the Zakura native header-sync service so a node can sync block headers over the Zakura transport ahead of downloading bodies.

Solution

• Bounded Zakura header-sync stream-5 wire messages and stateless header validation.
• Default network.zakura.header_sync config surface.
• Supporting zebra-state header storage / commit-flow changes (header-only commits that keep body availability separate from header availability).

Tests

• Header-sync unit tests + state header-range tests (zebra-state suite passes locally).
• 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 (notably merging the header-commit state flow with upstream's pruned-storage changes), commit organization, and this description.

PR Checklist

• Conventional-commit title
• Solution is tested
• Changelog updated

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Superseded by a 3-PR restructure: only commits at clean boundaries (C1/C5/C8) compile + pass clippy standalone, so the 5-way split was collapsed into #106 (P2P stack),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