Cache verified remote contiguous length

[marcus-pousette-hp]

Before:

read remoteContiguousLength
-> scan remote bitfield from stale cached position
-> return higher value
-> cache stays stale
-> next read scans same area again

After:

read remoteContiguousLength
-> scan remote bitfield once
-> cache the verified higher value
-> next read starts from the higher value

Local bench test from before and after patch.

10k reads over a 50M-bit verified contiguous extent
before: ~51.7 ms
after:  ~2.26 ms

PR was Co-authored with AI.