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Add the multi-bank pillar as Phase B (lettered, parallel to M-line and Phase D): CONTEXT.md §Multi-bank spec, PLAN.md Phase B block (B1 bank_book pure / B2 persist / B3 actions / B4 panel split), and docs/product/multi-bank.md framing. Additive only — BankIndex, the M0–M11 roadmap, and Phase D untouched.