docs(changelog): record the base-side failure count

Review follow-up: the entry covered the spread rename but not the other user-visible
change in the PR - a row whose base side failed, or whose means cannot be compared, now
reaches the warning total instead of sitting under a 'No significant change' summary.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: marius-bughiu

CHANGELOG.md

@@ -35,6 +35,7 @@ All notable changes to Celerity are documented here. This project follows [Keep
 - A benchmark shard no longer times out on a pull request that adds a benchmark class: the `main` base now replays the class list the PR head resolved, so shard *i* is the same slice on both sides. The job budget was also resized to the measured slices, which the suite had outgrown. Closes [#300](https://github.com/marius-bughiu/Celerity/issues/300).
 - A benchmark comparison that is missing a shard now says so in the PR comment, instead of reading exactly like a complete run. Closes [#300](https://github.com/marius-bughiu/Celerity/issues/300).
 - The PR benchmark comment no longer cries wolf: its noise guard is now a **3σ** bar over the two measurements' combined standard deviation, added in quadrature, rather than a 1σ sum. Replayed over a run whose library IL was byte-identical to `main`, that cuts flagged rows from **13 to 2** while detecting the same regressions on every benchmark precise enough to resolve one. Closes [#351](https://github.com/marius-bughiu/Celerity/issues/351).
+- A benchmark whose `main` side failed to measure, or whose two means cannot be compared at all, is now counted in that comment's warning total. Previously such a row appeared in the table while the summary line still read "No significant change vs main". Closes [#351](https://github.com/marius-bughiu/Celerity/issues/351).
 
 - `PartialSort.TopK` now throws `ArgumentException` when its `destination` overlaps its `source`, instead of silently returning a wrong answer and writing to the source it documents as untouched. Disjoint slices of one array are still accepted, matching `RadixSort` and `CountingSort`.
 - Corrected `RadixSort.ArgSort` XML documentation: only its `ReadOnlySpan<int>` overload rejects `indices` that shares storage with `keys`. Documentation only.