merge_batcher: weigh the geometric ladder by updates; split Merger::account

[frankmcsherry]

Two related changes to the MergeBatcher, lifted out of the chunk_basis work so they can land independently of the Chunk module that motivates them.

Weigh-by-updates. The geometric chain ladder previously compared chains by their chunk count (chain.len()). That is only a proxy for update count while every chunk is the same size; once a backend regrades — re-melding chunks so size and count decouple — a trickle of single-update chunks re-merges the head chain on every insert. Weigh chains by summed updates instead. A chain is immutable until merged, so its weight is computed once at push and cached alongside it (chains: Vec<(usize, Vec<Chunk>)>). Neutral for the existing vec backend (uniform chunk sizes make count and update-weight proportional); the behaviour change only bites a regrading backend.

Refocus the Merger trait. The bundled account() -> (records, size, capacity, allocations) splits into len() -> usize (update count — drives the ladder and the logger's records field) and a defaulted allocation() -> (size, capacity, allocations) for memory telemetry. The logger tuple is reassembled verbatim via a private record helper, so BatcherEvent's shape and the emitted figures are unchanged.

NOTE: breaking change for out-of-tree Merger implementors (e.g. Materialize) — rename account -> len, optionally override allocation. The only in-tree impl (vec::VecMerger) is migrated here.