feat(analytics): scenario engine + timeline/snapshot model

[kayodebristol]

Summary

Build the scenario comparison engine and temporal snapshot system.

Scenario Engine

• Define "what if" scenarios: cancel subscriptions, extra debt payments, income change
• Compare scenario vs baseline: impact on monthly burn, runway, debt payoff, net worth
• Scenarios are composable (combine multiple changes)

Timeline/Snapshot Model

• Monthly snapshots of full financial state
• Compare any two points in time
• Trend visualization data (design-dojo SVG primitives)

Acceptance

• Can create and compare scenarios
• Dollar impact computed correctly
• Monthly snapshots capture full state
• Timeline comparison works

[copilot-swe-agent]

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[kayodebristol]

@copilot Please implement this issue.