Select chains by cumulative proof of work

[webbananaunite]

Summary

• Replace the fixed four-block branch swap rule with cumulative proof-of-work comparison.
• Keep candidate branches extendable instead of capping them at the old swap depth.
• Search branch points by their absolute index and add regression tests for chain selection behavior.

Verification

• swift test --build-path /Users/yoichi/appOutput/blocks
• /Users/yoichi/.swiftly/bin/swift build -v --swift-sdk x86_64-swift-linux-musl --build-path /Users/yoichi/appOutput/blocks-linux-musl