fix(chartgen): name the backend when a drain slice's corpora disagree

[zannis]

What

The drain caption collected every in-slice corpus into one BTreeSet and rendered it flat:

…until 90 % of a corpus of 6M / 30k messages, published before they started, had been consumed

That names both sizes and attributes neither. A reader cannot tell which series drained which, which is strictly worse than naming one size or naming them all.

When every backend ran the same corpus the sentence is correct and stays exactly as it is. When they disagree, the sentence stops naming a size and a second caption line attributes them:

…until 90 % of its corpus, published before they started, had been consumed; no producer ran in the window

corpus differs by backend: inmemory 22k, kafka 2.2k — a smaller corpus is a shorter window, not a different measurement

Why now

It has never fired, because one pinned matrix means one corpus — MATRIX pins --drain-messages 6000000 for every backend.

The upcoming six-backend rerun is about to make it fire. SQS measures at ~900 msg/s against LocalStack, so the pinned corpus makes its pass ~51 h on this host (~17 h on a 3× faster one) to produce a 1.7-hour window measuring what a 30k corpus measures in seconds. The decision taken on that work is to run SQS on a smaller corpus, recorded as a documented deviation on the row and captioned on the chart.

The row half already works: drain.corpus is on every drain row, so the document records it by construction. The chart half is this PR — without it the deviation reaches the reader as an unattributed pair of numbers.

Scope of the rendering change

Every committed SVG still byte-matches. the_committed_results_document_renders_every_family (the svg == committed byte-compare) passes untouched, because both runs in the committed document drained identical corpora per payload leg:

backend	64 B	1 KiB	64 KiB
inmemory	6000000	3145728	49152
kafka	6000000	3145728	49152

Tests

• one_shared_corpus_is_stated_once_and_never_attributed — the uniform case keeps the flat sentence and adds no attribution line.
• a_backend_that_drained_a_smaller_corpus_is_named_in_the_caption — the deviating case names each backend's corpus, drops the flat claim, and keeps the drain rule.

One existing assertion moved from the raw SVG to the text nodes joined — the idiom this file already uses for caption prose ("wrapped text must keep every word"). The shorter sentence re-wraps at NOTE_WRAP, splitting no producer ran in the window across two <text> nodes; where a line happens to break was never part of the claim.

Verification

cargo fmt -- --check                                             # clean
cargo clippy --no-default-features --all-targets -- -D warnings  # clean
cargo nextest run --no-default-features                          # 334/334 passed

Depends-on: none — the caption change stands alone; the six-backend rerun that will exercise it depends on this, not the other way round.

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