feat(matlab): phased days x tDCS LME + Monte-Carlo power analysis

Adds tdcs_phase_lme (same LME refit within learning phases 0-5/6-10/6-13, with
per-phase slopes + interaction 95% CI) as phase_* switch cases, and
tdcs_power_sim (Monte-Carlo power for the early-phase interaction, scored by the
cluster-honest per-animal test and the LME test) + run_power. Findings: the
Box-B2 faster-acquisition signal is in the early phase; at n=3-5/group honest
power is 0.3-0.7 even at the observed effect (need ~8/group if effect is as
observed, ~20/group if half). Adds tPhasePower tests. Suite 35/35.

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

analysis/matlab/README.md

@@ -107,3 +107,24 @@ sensitivity sections — `report_anchor_only` (a two-anchor-only refit) and
 `report_mixed` (a Bayesian MAP random-intercept model) — are intentionally
 **not** ported: the anchor-only refit is closely tracked by the main-model H2
 term, and MATLAB's native `fitglme` already *is* the random-intercept model.
+
+## Phased learning analysis and power
+
+- **Phased days × tDCS LME** — the same `success ~ day*tDCS + (1|subject)` model refit within
+  learning phases (0–5, 6–10, 6–13), Box-B2 vs Box-A2, with per-phase slopes and the interaction
+  95% CI:
+  ```
+  matlab -batch "tdcs_glm('phase_mergeA2')"   # or phase_unmerged / phase_mergeB2
+  ```
+  The Box-B2 faster-acquisition signal sits in the early (0–5) phase; late phases converge. The
+  interaction p/CI use fitlme observation-level DF and are anticonservative at these small n
+  (see power analysis).
+
+- **Monte-Carlo power** — power to detect the early-phase interaction, scored with the
+  cluster-honest per-animal test and the LME test, across subjects-per-group and effect sizes:
+  ```
+  matlab -batch "run_power"          # default: mergeA2, phase 0–5
+  ```
+  Result: at our n (3–5/group) honest power is ~0.3–0.7 even at the observed effect; ~8/group
+  reaches ~80–90% if the effect is as large as observed, ~20/group if it is half that. The LME
+  test is anticonservative only at small n and converges to the honest test by n≈12.