compute: borrow each correction chunk once per merge, not once per update - #38896

`compute_correction_v2_chunk_size` sizes `CorrectionV2`'s staging area,
not its chunks: chunk bodies are fixed at the columnar ship size. The
staging size sets how many updates accumulate before a chain is minted,
and every chain minted is a chain some later read has to merge. Raising
the default from 8 KiB to 2 MiB, one chunk body's worth, takes insert on
a 4.19M-update append workload from 3503 ms to 1897 ms, with drain and
peak RSS unchanged. These are medians of three `correction_mem` runs
with the rest of the stack applied and no pool installed. The curve is
monotone across the range (the new `bench_chunk_size` sweep in the
`correction` bench shows it), so the default is bounded only by the
per-sink, per-worker cost of the staging vector.

The stage counted its fill in tuples of `size_of::<(D, Timestamp,
Diff)>()`, which is 40 bytes for a `Row` however wide the row is, so a
byte-sized flag bounded neither the staging memory nor the reported size
once rows spill out of line. A new `DataBytes` trait on the buffer's
data types reports each datum's bytes with the heap included (`Row` via
`byte_len`, `DataflowErrorSer` via its serialized length), the stage
ships when the staged bytes reach the flag, and its size metrics carry
the same figure.

`stage_ships_by_bytes` checks that rows of two widths ship after the
number of updates their bytes predict.

That vector was allocated eagerly at the configured capacity and
re-allocated at that capacity on every flush. At 2 MiB per sink and
worker that is no longer free, so it now grows on demand and gives its
allocation back on flush.

The mzcompose and parallel-workload flag tables follow the new default,
since CI runs the value listed there rather than the compiled default.
The flag's description now says what it sizes.

First of four, split out of #38645: this, the O(1) metrics (#38895), the
per-chunk borrow hoist (#38896), and the pool substrate (#38897) land in
that order. Supersedes the fork PR #38647.

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Co-authored-by: Claude Fable 5.1 <[email protected]>