Bridge verified BNB dividends into Earn accounting

[Pigbibi]

Problem

Binance Earn forward conservation rejected a verified positive BNB balance delta because the existing accounting only accepted realtime rewards, known fills, and USDT deposits. The wallet diagnosis could show the dividend row, but no production consumer could safely consume it.

Change

• Add a bounded, read-only asset/assetDividend adapter for BNB rows in (start, end].
• Require exact BNB asset, positive Decimal amount, integer direction == 1, valid id/tranId/time, no duplicate identity, and a complete non-full page.
• Bridge the verified quantity through the existing Earn conservation path in portfolio preparation, prospective recovery's two samples, and funding cash review. The funding review leaves the checkpoint unchanged; formal preparation consumes the quantity once through the existing owner writer.
• Keep the dividend out of daily_external_principal_usdt and extra PnL. Preserve the diagnostic distinction between an empty missing-total dust response and non-empty/incomplete dust data.

Validation

• PYTHONPATH=. uv run --no-sync pytest -q — 884 passed, 91 subtests passed.
• uv run --no-sync ruff check ... — passed.
• git diff --check — passed.

No live account read, order, Earn subscription/redemption, runtime enablement, or production state mutation was performed.