feat(log): expose the fx pnl component in the financial data log

[TaprootFreak]

Summary

totalBalanceChf (the operating equity) moves through transactional yield, price drift on open positions (FX) and errors. The FX component has not been visible anywhere - it was only derivable as a residual between the change log and the balance delta.

Every FinancialDataLog entry now carries balancesTotal.fxPnlChf: the price effect since the previous snapshot, computed as the previous net position times the price change, summed over all financialType assets. With it, every equity move decomposes into yield + fx + rest.

• purely additive: no existing field, the valid-flag chain and the safety mode are untouched
• the first entry (no reference point) omits the field; the dashboard maps missing values to zero, so historic entries stay readable
• positions absent from either snapshot contribute nothing (new positions enter via flows, not FX)

Test plan

• unit tests: long/short positions with rising/falling prices, financialType filter, no-predecessor case, appearing/disappearing assets, dashboard zero-default
• full jest run green, format:check / lint / type-check clean

[TaprootFreak]

Both review tracks (conventions + correctness) came back clean on the first pass. Verified explicitly: the price-effect decomposition (previous net position times price change, quantity effect stays in the yield residual, no cross term), the skew-safe reference point (the predecessor is the last valid entry, matching how the chart consumes the series), the financialType filter parity with the balance buckets (synthetic RefCredit contributes nothing), and single-parse reuse of the predecessor message in the every-minute job.