docs(matlab): label tDCS main effect as Day 1 (= our day 0), matching the paper

The model already evaluates the tDCS main effect at day 0, which is the paper's
'Day 1'; relabel the report/docstring to state the equivalence explicitly so the
'equal on Day 1' comparison is unambiguous. No model change (re-indexing day
would move the main-effect evaluation point off Day 1). Suite 31/31.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

analysis/matlab/tdcs_lme.m

@@ -8,8 +8,11 @@ function L = tdcs_lme(S, cfg)
 %
 %   replicating the published formulation (a linear mixed-effects model for the
 %   number of successful reaches with a days x tDCS interaction, a main effect
-%   of days, and a main effect of tDCS). `day` is used raw (not centered), so
-%   the tDCS main effect is the group difference on the first day ("Day 1").
+%   of days, and a main effect of tDCS). `day` is raw and 0-indexed, and OUR
+%   day 0 corresponds to the paper's "Day 1", so the tDCS main effect is the
+%   group difference on Day 1 -- directly comparable to the paper's "equal on
+%   Day 1" test. (Do not re-index day to 1-based: that would move the main
+%   effect's evaluation point off Day 1.)
 %
 %   L fields:
 %     .lme         the LinearMixedModel object