Task: Close remaining FX rate-display gaps in Qt

CrmCrossRatesMatrixMdiWindow's main grid cells are already convention-aware (via crm_rate_formatter), but that fix was scoped to the CRM matrix only. Every other FX rate-display surface still hand-formats:

• CurveSnapshotMdiWindow (Grid/History) — hardcoded \'f\=, 6=; its Δ (bp) column hardcoded \'f\=, 1= (this column is correct to keep in bp — see story Goal — it just needs the same convention-driven precision the level column gets).
• CrmCrossRatesMatrixMdiWindow's own overview panel (selected-pair label + sparkline) — hardcoded \'f\=, 5=, missed by the original wiring task.
• SamplePricePathsChart axis labels — QtCharts default format, no precision awareness.
• FxSpotGenerationConfigDetailDialog=/=HistoryDialog, MarketSimulatorWindow summary panel — bare QString::number(double).

Build a small, reusable presentation-layer formatter (mirroring crm_rate_formatter's shape but generic to any currency_pair_convention-keyed rate, not CRM-specific data) and wire each site above to resolve the relevant pair's convention (via currency_pair_convention_cache, the same cache the CRM wiring already uses) and format through it.

• A generic, convention-aware rate formatter exists (stateless, presentation-layer, DB/NATS-free, matching crm_rate_formatter's contract) usable by any currency_pair_convention-keyed surface, not just CRM cells.
• CurveSnapshotMdiWindow, CrmCrossRatesMatrixMdiWindow's overview panel, SamplePricePathsChart, FxSpotGenerationConfigDetailDialog=/=HistoryDialog, and MarketSimulatorWindow's summary panel all resolve the relevant pair's convention and format through it, replacing every hardcoded \'f\=/\='g\=, N= call site listed in the Goal.
• CurveSnapshotMdiWindow's Δ (bp) column keeps showing deltas in bp (not converted to the level's own precision) — only the level column's precision becomes convention-driven.
• Real Catch2 coverage of the new formatter, independent of any Qt/NATS harness.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.