MINOR: cache: track full and hints entries in per-pool LRU lists

MINOR: cache: track full and hints entries in per-pool LRU lists

Introduce two LRU lists in struct cache (full_lru and hints_lru) and an
"lru" link in struct cache_entry, plus a CACHE_EF_STRIPPED flag to
mark hints-only entries. A new cache_row_reattach() helper places each
entry in the appropriate LRU when a row is returned to the cold pool.

No behaviour change yet: the lists are populated but no code consumes
them. The next commit uses them to drive early hints-aware eviction.