compute: borrow each correction chunk once per merge, not once per update - #38649

`CorrectionV2::update_metrics` walked every chain and every chunk on every
operation, from four call sites including insert, so its cost grew with the
buffer: 17% of insert at 16.8M updates and rising. An `Accounting` handle,
shared by the buffer and by every structure that holds its chains, now carries
running record, size, and allocation totals, so a metrics update reads three
counters.

The totals hook the discipline the introspection logging already kept: a chain
is announced once when the buffer takes it and retired once before it is dropped
or consumed, so the totals and the logging cannot drift apart. Each chunk caches
its serialized size at construction, and each chain maintains the sum as chunks
are pushed, so no accounting step pages a chunk in.

The reported size is the logical serialized size of the bodies, resident or not.
The previous walk reported resident bytes only, which depended on what the pager
had done with each chunk since. Residency belongs to the pager, which changes it
without telling the buffer. Capacity is reported equal to size, since chunk
bodies are exactly sized when minted, and allocations count one per chunk plus
the staging vector while it holds one.

`metrics_totals_match_walk` checks the totals against a walk after inserts, a
partial drain, a since advance, and a consolidation.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

…date

Every element access in `CorrectionV2` re-decoded its chunk. `Chunk::index` was
`self.column().borrow().get(i)`, and `Column::borrow` on a serialized column
rebuilds the struct-of-arrays view from the serialized header, so a k-way merge
paid O(log k) header decodes per output update: 42.7% of an insert profile,
ahead of the merge itself at 12.2%.

Borrowing is now hoisted to one view per chunk, or per chunk pair in the two-way
merge, whose inner loop runs until either cursor crosses a chunk boundary.
`Cursor` gained `chunk_handle`, `reads_from`, and `get_with` for callers that
hold a view across cursor steps. `get` and `index` remain the documented
single-access forms. Chain iteration, chunk splitting, and the time search
index a hoisted view likewise.

Holding a view inside the iterator that `updates_before` hands across the
persist writer's `await` needs the view to be `Send`, which is a new bound on
the container. The container bounds move into a `DataContainer` helper trait
so `Data` stays readable.

Insert improves 1.6x on a 4.19M-update append workload, and 2.0x under a memory
cap, where each redundant decode also faulted the header page back in.

Co-Authored-By: Claude Fable 5.1 <[email protected]>