M1.2 Headers-first sync to tip with full PoW + difficulty validation

[melvincarvalho]

Part of #1 (M1).

Goal

Using the engine's HeaderEngine, sync all mainnet headers to chaintip via the bridge (#? M1.1), validating prev-link, proof of work, timestamps, version rules, and the 2016-block retarget.

Benchmark / acceptance

• Sync mainnet headers to tip; report total time and peak memory.
• validateChain passes for the entire chain.
• Tip hash matches a public block explorer.

Notes

No signatures here, so it is fast and honest: it is real consensus validation of the header chain. This is the Tier 0 backbone.

[melvincarvalho]

Done on testnet4 (the agreed first target; mainnet is a network-param flip).

src/sync-testnet4.js connects to a live testnet4 peer over TCP, syncs the header chain from genesis with a per-batch connectivity check, and fully validates it via HeaderEngine.validateChain (PoW, difficulty, BIP 94 timewarp, the 20-minute min-difficulty walk-back).

Measured: ~140k headers downloaded in ~3-9s, all valid, tip confirmed by 2/2 independent p2p peers (no third-party API). Validation runs ~15-21k headers/sec on the pure-JS engine, single core.

Acceptance met: validateChain passes for the whole chain; tip cross-checked. Resume + reorg + most-work across peers continues in #9 (M1.3).