Add Phase 6.5A: same-batch linesearch REVISES Phase 6A conclusion

Phase 6A's "better credit → worse loss" was a protocol artifact caused by:
1. Credit normalization (inflated DFA, suppressed Vec magnitude ordering)
2. Held-out evaluation (measured generalization failure, not exploitability)
3. Gradient clamping

With strict same-batch evaluation:
- Oracle BP: dL_same = -0.406 (strongest descent)
- Vec_M4: dL_same = -0.135
- ScalarCB: dL_same = -0.025
- DFA: dL_same = -0.003
Same-batch loss decrease is MONOTONIC with credit quality.

But held-out loss INCREASES for all non-DFA methods (Case D: overfitting).
The bottleneck is batch-level generalization, not surrogate exploitability.

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

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