What

Add support for multi-valued tables in turbo-persistence.

A multi-valued table allows multiple distinct values to be associated with a single key. Each family is independently configured as SingleValue (existing behavior) or MultiValue via the new FamilyKind enum.

Why

This will support the TaskCache table (implemented in #88904), where keys will change to be hashes instead of full TaskType values. This greatly decreases DB size and speeds up queries due to smaller key sizes, at the cost of hash collisions requiring multiple values per key.

How

API

• New FamilyKind enum (SingleValue / MultiValue) and per-family FamilyConfig in DbConfig
• get() for single-valued families (panics if called on multi-valued)
• get_multiple() for multi-valued families, returns SmallVec<[ArcBytes; 1]> — stack-allocated for the common 0–1 result case, heap-scales when needed
• put() and delete() are unchanged — the family kind controls dedup/compaction behavior

Write path & compaction

• Single-valued (unchanged): last-write-wins per key
• Multi-valued: all values are maintained, deletions 'shadow' old values.
• Deletion inserts a tombstone that shadows all older values for that key across SST layers. Values written after the tombstone in the same batch are retained.
• To avoid extra buffering logic the MergeIter semantics were changed so it produces 'newest' entries first
  • for SingleValues families this is no different since we only keep one value
  • for MultiValued families this makes dealing with tombstones trivial, but does mean compaction will reverse the order of the set. For this reason we make no guarantees about ordering.

Read path

• Controlled by a FIND_ALL const generic on the internal lookup methods
• Single-valued (FIND_ALL=false): binary search, return last match, stop
  • This fixes a bug where we might return Deleted when there is a value in the SST depending on what the search algorithm found first
• Multi-valued (FIND_ALL=true): scan all matching entries in the SST block, then continue to older SSTs. If a tombstone is found, stop searching older layers.