Add Phase 10A.8C: 3-seed replication — scaffold gains are marginal

3-seed results (mean±std):
- DFA: 0.306±0.006
- perlayer_vector α=0.75: 0.304±0.006 (-0.2%, not significant)
- random_trainable α=0.75: 0.313±0.007 (+0.7%, marginal, error bars overlap)

Single-seed gains (+1.1% perlayer, +0.8% vec) do not robustly replicate.
The scaffold mechanism provides at best a marginal, statistically uncertain benefit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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@@ -647,6 +647,16 @@ But 28.5% < DFA 31.2% → continuous trainability adds ~2.7% additional value.
 Both methods optimal at α=0.75. perlayer_vector (+1.1%) ≈ random_trainable (+0.8%).
 Per-layer vector is the minimal sufficient scaffold.
 
+### Phase 10A.8C: 3-Seed Replication
+
+| Method | final acc (3 seeds) | diff vs DFA |
+|--------|---------------------|-------------|
+| DFA | 0.306±0.006 | baseline |
+| perlayer_vector α=0.75 | 0.304±0.006 | -0.2% (not significant) |
+| random_trainable α=0.75 | 0.313±0.007 | +0.7% (marginal) |
+
+Single-seed gains do not robustly replicate. Error bars overlap.
+
 ### Experiment IDs (Phase 10)
 - `prefit_threshold/`: Phase 10A prefit threshold curve
 - `blend_dissection/`: Phase 10A.5 blend mechanism dissection