docs: state the metric-symmetry rule rather than how it was arrived at

CPU_DATA_ANALYSIS describes how the pipeline works; the account of which bar was
tried first belongs in the commit that changed it. Same facts, no narrative.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/CPU_DATA_ANALYSIS.md

@@ -403,9 +403,9 @@ The output includes:
 
 This stage provides centering information used for systematic absences in prediction (§8.4) and for reporting.
 
-**A metric symmetry has to earn itself.** The class is chosen from the *unrefined* candidate against a fixed angular tolerance (3°), so a lattice that is pseudo-symmetric to a few tenths of a degree is promoted a class too far — and the constraint then snaps a real angle to the ideal one, which throws nearly every reflection of every frame outside tolerance. Measured on a monoclinic crystal pseudo-C-orthorhombic to 0.42°, the promoted cell indexed 2 of 60 frames where its own primitive cell indexed 39: the same lattice, $\mathbf{b}_{oC}=-(\mathbf{a}+2\mathbf{c})$, at exactly twice the volume. Note the direction of the trap — **more accurate candidates make it worse**, because a run escapes only when the raw candidate is inaccurate enough to miss the promotion window.
+**A metric symmetry has to earn itself.** The class is chosen from the *unrefined* candidate against a fixed angular tolerance (3°), so a lattice that is pseudo-symmetric to a few tenths of a degree is promoted a class too far — and the constraint then snaps a real angle to the ideal one, which throws nearly every reflection of every frame outside tolerance. Measured on a monoclinic crystal pseudo-C-orthorhombic to 0.42°, the promoted cell indexed 2 of 60 frames where its own primitive cell indexed 39: the same lattice, $\mathbf{b}_{oC}=-(\mathbf{a}+2\mathbf{c})$, at exactly twice the volume. A more accurate candidate is more likely to be promoted, not less: a run escapes the promotion only when the raw candidate misses the tolerance window.
 
-For rotation data the first pass therefore refines the constrained cell *and* an unconstrained (triclinic) one from the same spots — which it finds itself, over a sample spread across the sweep, rather than reading what the acquisition wrote — and settles the two on how many of a fixed set of validation frames each actually indexes. The bar is a clear majority rather than a margin: an unconstrained refinement holds no cell parameter fixed, so it can only index at least as many frames, and on genuine symmetry it does index a few more. Only a constrained cell that fails outright while its unconstrained cell works is evidence of a false promotion. That asymmetry is what keeps real symmetry — a 10 % margin, tried, demoted a genuine $I$-centred orthorhombic lattice to $P1$. The intensities settle the space group later regardless (§13).
+For rotation data the first pass therefore refines the constrained cell *and* an unconstrained (triclinic) one from the same spots — which it finds itself, over a sample spread across the sweep, rather than reading what the acquisition wrote — and settles the two on how many of a fixed set of validation frames each actually indexes. The bar is a clear majority, not a margin. An unconstrained refinement holds no cell parameter fixed, so it can only index at least as many frames as the constrained one, and on genuine symmetry it indexes a few more — a percentage margin therefore demotes real lattices (measured: a genuine $I$-centred orthorhombic to $P1$). Only a constrained cell that fails outright while its unconstrained cell works distinguishes a false promotion. The intensities settle the space group later regardless (§13).
 
 **Note.** In ambiguous or special cases, forcing space group to $P1$ (no symmetry assumptions) is recommended.