docs: changelog for the beam-stop outward-background fix

docs/CHANGELOG.md

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 This is an UNSTABLE release. It includes many experimental features, as well as many AI generated fixes. We recommend using rc.152 for production use.
 
 * rugnux: New **beam-centre measurement before indexing** (`--estimate-beam-center`), which takes the direct beam from the symmetry of the spots where the sweep reaches at least half a turn, from the radial background profile where it does not, and keeps the value in the file where neither can measure it; the rotation axis' skew about the beam is fitted at the same time (`--no-fit-spindle` keeps the axis from the file).
+* Beam-stop detection: the outward background a ring is compared against is now taken only over rings holding enough pixels to be measured, so a single reflection recorded in a nine-pixel corner ring can no longer raise the threshold and have the detection declare everything out to about 13 Å to be inside the beam stop.
 * Lattice search: a reduced cell whose β sits on the Niggli type boundary now keeps its centring instead of falling through to triclinic, so a centred lattice is no longer read as primitive according to which way its refined cell happened to reduce.
 * rugnux: The second rotation pass now reuses a space group that came from the intensity-based centred-lattice test, as it already did for every other group, instead of re-deciding the symmetry at the refined geometry.
 * Space-group search: where the search on the well-measured observations (`--search-min-zeta`) and the search on all of them disagree, the answer is now always the one the merge of all the observations supports, instead of whichever of the two found more symmetry.