refactor: add SeriesBufferInner type

[KevinWMatthews]

This PR moves two fields of SeriesBuffer into a new type, SeriesBufferInner, the moves this type to a module inner. This inner type provides/controls access to:

• the points buffer (write, mutex-guarded)
• the count (read-only, atomic)

The inner module restricts direct access to these struct members, requiring access through SeriesBufferInner's API.

The intent is for:

• updates to the points buffer can be slow and must be coherent
• reads / checks to the points are fast (do not require locking)

For the motivation behind this PR, see #139.