feat: add derivative what-if previews

[felipecsl]

Context

Issue #48 adds the explicit non-submitting broker gate needed to validate spread acceptance, margin, fees, and warnings without reaching placement.

What changed

• adds spread preview for one exact atomic vertical and broker doctor --trading
• requires exact account selection and masks account values in output
• keeps user-facing credit/debit positive while the adapter handles IBKR signed encoding
• creates a material-term preview digest with a five-minute expiry
• binds previews to account/environment, exact contracts and ratios, quantity, limit, TIF, session, timestamp, and normalized What-If result
• rejects expiry and account/environment mismatch and treats What-If as authoritative over advisory permissions
• consumes @huskly/ibkr-client@^0.10.0

Safety

Preview output explicitly states no order was submitted. The application capability exposes no placement, warning acknowledgment, cancellation, or raw broker request path.

Validation

• yarn run check
• yarn build
• 10 test files passed

Closes #48