Add bounded BNB wallet evidence to Earn diagnosis

[Pigbibi]

Problem

The bounded earn-forward-diagnose run can leave a positive BNB residual unexplained even when Spot/Flexible Earn, realtime counters, trades, and external flows are stable. The existing BNB wallet activity helper only returned counts, so one diagnostic run could not compare the already approved assetDividend and Spot dribblet surfaces.

Change

• Keep diagnose_bnb_wallet_activity backward compatible and optionally retain its two response row sets in memory for the new diagnostic consumer only.
• In earn-forward-diagnose, call those two GETs only for a positive BNB residual when the other bounded checks are stable.
• Validate the checkpoint-to-now window, row uniqueness, Decimal amounts, BNB targetAsset, and aggregate shape without exposing amounts or source IDs.
• Report exact dividend, transfer, fee-adjusted, and combined comparison flags, sanitized source failure metadata, and a separate one-eight-decimal-unit residual flag. Fee and dividend net semantics remain explicitly unverified; all causal and execution flags remain false.
• Add bounded regression coverage and operator documentation.

Validation

• .venv/bin/python -m pytest -q — 855 passed, 91 subtests passed.
• .venv/bin/ruff check application/broker_reconciliation.py scripts/migrate_daily_accounting_state.py tests/test_balance_flow_diagnostics.py tests/test_earn_forward_diagnosis.py — passed.
• No remote workflow was dispatched by this change.