feat(core): add skiplist to CheckPoint for faster traversal

feat(core): add skiplist to CheckPoint for faster traversal

Adds a skip pointer and an index field to CheckPoint to accelerate
get(), floor_at(), and range(). push() and insert() maintain the
index/skip invariants on the rebuilt chain.

The skip pointer follows Bitcoin Core's `CBlockIndex::pskip` pattern,
adapted to operate on checkpoint indices (not heights) so it works on
sparse chains. Skip distances grow exponentially as you walk back,
yielding O(log n) traversal.