`put_newer` crashes with "not a valid match specification" when the key contains a map

[dfalling]

When PartitionedBuffer.Map.put_newer/4 is called twice for the same key within one buffer flush interval and that key contains a map, the second call raises:

** (ArgumentError) errors were found at the given arguments:
  * 2nd argument: not a valid match specification
    (stdlib 7.2) :ets.select_replace/2
    (partitioned_buffer 0.4.2) lib/partitioned_buffer/partition.ex:138

Root cause

In replace_match_spec/3 (lib/partitioned_buffer/partition.ex), the match head embeds key raw, while the body wraps it via ms_literal/1:

[
  {
    entry(key: key, value: :_, version: :"$1", updates: :"$2"),  # head — raw key
    [{:>, version, :"$1"}],
    [
      {entry(
         key: ms_literal(key),       # body — wrapped key (maps → {:const, map})
         ...
       )}
    ]
  }
]

If key is {:foo, [%{type: :bar}]}, the head contains a raw %{type: :bar}, which ETS does not accept in match-spec heads — raw maps are not matchable. The spec is rejected before any matching happens.

Example

{:ok, _} = PartitionedBuffer.Map.start_link(name: :buf, processor: fn _ -> :ok end)

key = {:user, %{id: 1}}
:ok = PartitionedBuffer.Map.put_newer(:buf, key, "v1", 1)
:ok = PartitionedBuffer.Map.put_newer(:buf, key, "v2", 2)  # crashes

The first call goes through :ets.insert_new/2 (succeeds). The second falls into the select_replace branch with the bad spec.

Workaround

We're working around it by hashing the map portion of our cache keys before storing.