Plan advanced math-engine extension set (distributions/inference/calculus/number theory)

## Goal
Create an incremental roadmap for higher-order math features documented in Desmos broader function sets.

## Scope
- Define phased implementation plan for:
- probability distributions and sampling helpers
- inference tests and result properties
- calculus operators (`exp`, derivative, integral, sum, product)
- extended number theory/hyperbolic trig functions
- Identify parser/evaluator architecture changes required for each phase
- Define performance and precision expectations per feature class

## Acceptance Criteria
- A written RFC/design doc is linked in this issue.
- Features are prioritized into milestones with clear dependencies.
- Each milestone has test strategy and rollback plan.