Remove sample identities from the repository; document the rule

Datasets may be confidential; sample names and measured unit cells committed to
the repo can leak outside the group working on them. Scrub existing occurrences
and add a "No sample identities in the repository" section to CLAUDE.md
(forbidden: sample/dataset names, internal codes, measured cells tied to a
sample; fine: space group / lattice / twinning descriptors).

- Comments: replace internal dataset codes and protein names with the
  crystallographic situation they illustrate (centred vs pseudo-symmetric,
  holohedral, cubic, F-cubic/hexagonal, ...).
- Docs: same, in the analysis/writer/stream references and example configs.
- Tests: rename sample-named identifiers, TEST_CASE names, file prefixes and
  asserted labels to neutral crystallographic names (e.g. tetragonal_uc);
  behaviour unchanged. Reduce the CrystFEL reference PDB to a bare CRYST1 cell
  file (cell.pdb) and rename the reference data file.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

image_analysis/LoadFCalcFromMtz.cpp

@@ -38,7 +38,7 @@ const gemmi::Mtz::Column* SelectDefaultColumn(const gemmi::Mtz& mtz, bool& squar
         }
     // Fall back to a plain structure-factor amplitude (type F): a deposited/observed FP or FOBS, or a
     // lone FC, squared to an intensity like F-model. Lets a reference MTZ that carries only amplitudes
-    // (no F-model or IMEAN/I - e.g. the KR2 8cl8 deposition, which has only FP) still seed CCref
+    // (no F-model or IMEAN/I - e.g. a deposition that has only FP) still seed CCref
     // without an explicit --reference-column.
     for (const char* label : {"FP", "FOBS", "F", "FC", "Fobs", "F-obs"}) {
         if (const auto* col = mtz.column_with_label(label, nullptr, 'F')) {