M1.5 Benchmark: end-to-end Tier 0 headers sync (phone + desktop)

[melvincarvalho]

Part of #1 (M1). Closes Tier 0 foundation.

Goal

Measure the whole Tier 0 headers path end to end across device classes.

Acceptance

• A table of measured numbers (time to tip, storage used, peak memory; battery if feasible) on at least a phone and a desktop.
• Confirms the Tier 0 claim: headers + light foundation usable in minutes, runs on a phone.
• Feeds the revised tier model in M0.5 / Roadmap: browser-native Bitcoin node (epic) #1.

[melvincarvalho]

Desktop numbers captured (this dev machine): header validation ~15-21k headers/sec pure-JS single core; download ~16-44k headers/sec over the network; full testnet4 header chain (~140k) end to end in ~10-20s including validation. See bench/headers-validate.js and src/sync-testnet4.js.

Still to do: run on a phone, capture storage/memory/battery, and confirm the Tier 0 "minutes on a phone" claim.