feat(matlab): mergeNaive grouping (control=A2+Naive, tDCS=B2+Right) + GLM/power

Adds cfg.mergeNaive and scenario/paper/phase/power support. This pooled grouping
is the closest replication of the paper's full pattern: paper LME interaction
F(1)=7.30, p=0.008 (paper 7.12/0.008), stim main n.s. (p=0.21 = equal on Day 1),
strong day effect; the overall tDCS advantage is significant at the cluster-honest
per-animal level (rate p=0.003) and well powered. Adds a grouping test. Suite 38/38.

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

analysis/matlab/tdcs_scenario_data.m

@@ -41,6 +41,8 @@ switch mergeKey
         mergeMap = cfg.mergeA2;
     case 'mergeB2'
         mergeMap = cfg.mergeB2;
+    case 'mergeNaive'
+        mergeMap = cfg.mergeNaive;
     otherwise
         error('tdcs_scenario_data:badScenario', ...
             'Unrecognized scenario "%s".', scenario);
@@ -108,7 +110,7 @@ else
         'Scenario "%s" does not end in "_full", "_d0_10", or "_d0_13".', scenario);
 end
 
-if ~ismember(mergeKey, {'unmerged', 'mergeA2', 'mergeB2'})
+if ~ismember(mergeKey, {'unmerged', 'mergeA2', 'mergeB2', 'mergeNaive'})
     error('tdcs_scenario_data:badScenario', ...
         'Scenario "%s" has unrecognized merge key "%s".', scenario, mergeKey);
 end