Add Phase 10A: no prefit threshold — even random Vec blend beats DFA by +1.3%

E_prefit=0 (random Vec) + blend(0.75): 32.4% vs DFA 31.1% (+1.3%)
E_prefit=15: 32.3% (+1.2%)
E_prefit=60: 32.5% (+1.4%)

Frozen Gamma/rho near zero at all prefit levels. The Phase 9A success was NOT
from Vec learning useful credit — it was from the blend mechanism itself providing
regularization/diversification over pure DFA.

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

1 files changed, 23 insertions, 1 deletions

diff --git a/NOTE.md b/NOTE.md
index f07546c..e6ba41c 100644
--- a/NOTE.md
+++ b/NOTE.md
@@ -5,7 +5,7 @@
 - **pilot**: Controlled iteration (commits 0b9ebb2, 7baf7ae)
 - **frozen**: Code at commit 0b9ebb2 for all reported results
 
-## Status: PHASE 9 COMPLETE — OFFLINE PREFIT + BLEND IS THE KEY
+## Status: PHASE 10A — NO PREFIT THRESHOLD, BLEND ITSELF IS THE ACTIVE INGREDIENT
 
 ---
 
@@ -550,3 +550,25 @@ The +1.5% gain from 9A's blend(0.75) at t0=5 is the project's best online result
 - `checkpointed_handoff/`: Phase 9A checkpointed handoff with branches
 - `periodic_refit/`: Phase 9B periodic refit
 - `topdown_curriculum/`: Phase 9C top-down curriculum
+
+---
+
+## Phase 10A: Prefit Threshold Curve
+
+**Setup**: t0=5, blend_075, E_prefit in {0, 15, 60}, seed=42
+
+| E_prefit | Gamma_frozen | rho_frozen | final acc | diff vs DFA |
+|----------|-------------|-----------|-----------|-------------|
+| 0 (random Vec) | -0.005 | 0.014 | **0.324** | **+1.3%** |
+| 15 | 0.002 | 0.011 | 0.323 | +1.2% |
+| 60 | -0.001 | -0.009 | 0.325 | +1.4% |
+| continue_DFA | — | — | 0.311 | baseline |
+
+**Case C: NO prefit threshold exists.** Even random Vec (E=0) with blend(0.75) beats DFA.
+
+**Critical reinterpretation of Phase 9A**: the +1.5% gain was NOT from Vec learning good credit.
+Frozen Gamma/rho are near zero at all prefit levels. The benefit comes from the blend mechanism
+itself — blending DFA with any additional signal provides regularization/diversification.
+
+### Experiment IDs (Phase 10)
+- `prefit_threshold/`: Phase 10A prefit threshold curve