[WIP] Build deterministic financial computations in analytics package

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat(analytics): deterministic financial computations</issue_title>
<issue_description>## Summary
Build packages/analytics — deterministic financial analysis that sits on top of the ledger and resolution layers.

Owns

• Trend analysis: Spending trends by category/merchant over time
• Debt scenarios: Payoff timelines, interest projections, snowball vs avalanche
• Liquidity runway: "How many months can I sustain current burn?"
• Budget vs actual: Set budgets per category, compare against actuals
• Monthly burn: Total outflow analysis with recurring vs discretionary split
• Recurring commitment load: Total monthly obligated spend (subscriptions + bills + debt payments)
• Variance analysis: Month-over-month and category-level variance detection

Principles

• All computations are deterministic — no AI, no randomness
• Inputs: ledger postings + resolved transactions + time range
• Outputs: structured data objects, not prose
• Every number is traceable to source postings

Key Computations

// Monthly burn rate
computeMonthlyBurn(accountId, period): { total, recurring, discretionary, byCategory }

// Liquidity runway
computeRunway(accounts, burnRate): { months, pessimistic, optimistic }

// Recurring load
computeRecurringLoad(resolvedTransactions): { monthly, annual, items[] }

// Budget variance
computeVariance(budget, actuals, period): { overBudget[], underBudget[], net }

Acceptance

• Monthly burn calculation is accurate against known test data
• Budget vs actual comparison works
• Recurring load correctly sums detected subscriptions
• All outputs include source transaction references</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes feat(analytics): deterministic financial computations #55

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[kayodebristol]

Closing this PR to enforce the 1-PR-per-repo rule. Only one Copilot PR is allowed per repository at a time. The oldest PR (#48) will be kept active.