Add Phase 7A: snapshot time sweep shows early snapshots have positive held-out transfer

At epoch 5 (acc=49%), Vec_M4 5-step: dL_held=-0.005 (PUR=0.70)
  Oracle BP 5-step: dL_held=-0.009 (PUR=1.05)
  DFA 5-step: dL_held=+0.003 (always hurts held-out)

By epoch 20, generalization window closes. Held-out failure is late-snapshot artifact.
Better credit → lower update variance (Vec=0.8 vs DFA=40), not higher.

Key implication: DFA warmup delays credit bridge past its useful window.
Credit should be used from epoch 0, not after 20% warmup.

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

1 files changed, 32 insertions, 1 deletions

diff --git a/NOTE.md b/NOTE.md
index a57fd30..892cf1e 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 6.5 PROTOCOL AUDIT — PHASE 6A CONCLUSION REVISED
+## Status: PHASE 7A SNAPSHOT TIME SWEEP — EARLY SNAPSHOTS SHOW POSITIVE TRANSFER
 
 ---
 
@@ -418,3 +418,34 @@ gradient noise) could make better credit usable.
 ### Experiment IDs (Phase 6.5)
 - `exploit_linesearch/`: Phase 6.5A smoke test (Oracle + Vec, last1, raw)
 - `exploit_linesearch_full/`: Phase 6.5A full sweep (all methods, ranges, norm modes)
+
+---
+
+## Phase 7A: Snapshot Time Sweep
+
+**Setup**: BP snapshots at epoch {5, 20, 100} (acc 0.49/0.57/0.62).
+Train Vec_M4 on each frozen snapshot. Test 1-step and 5-step with raw credit, last-block-only.
+
+**KEY FINDING: Held-out failure is primarily a LATE-SNAPSHOT artifact.**
+
+5-step DeltaLoss held-out:
+
+| Epoch | DFA dL_held | Vec dL_held | Oracle dL_held | Vec PUR |
+|-------|-------------|-------------|----------------|---------|
+| **5** | +0.003 | **-0.005** | **-0.009** | **0.70** |
+| 20 | +0.001 | +0.002 | +0.000 | -3.87 |
+| 100 | +0.000 | +0.001 | -0.001 | -1.01 |
+
+At epoch 5: Vec decreases held-out loss (PUR=0.70), Oracle too (PUR=1.05).
+DFA INCREASES held-out at all snapshots.
+
+By epoch 20 the generalization window closes.
+
+**Better credit produces MORE consistent updates** (Vec variance=0.8 vs DFA variance=40).
+The problem is not batch-specificity but snapshot timing: credit is useful early, useless late.
+
+**Implication**: The DFA warmup (which delays credit bridge to epoch ~20) is counterproductive.
+Credit bridge should be used from epoch 0.
+
+### Experiment IDs (Phase 7)
+- `snapshot_time/`: Phase 7A snapshot time sweep with BP checkpoints