feat(go): add 14 formula functions (stats, random, modern logic, XLOOKUP)

[Mukbeast4]

Summary

Adds 14 spreadsheet functions to the formula engine:

Category	Functions
Statistics	`MEDIAN`, `STDEV`, `STDEVP`, `VAR`, `VARP`, `RANK`, `LARGE`, `SMALL`, `PERCENTILE`
Random	`RAND`, `RANDBETWEEN` (uses `math/rand/v2`)
Modern logic	`IFS`, `SWITCH`, `XLOOKUP` (exact match + default)

Why

Brings the function count from ~33 to ~47, closing the gap with Excel/Calc for common analytical workloads.

Test plan

• `go test ./...`
• `go vet ./...`
• `gofmt -s -l .` (empty)
• 19 new tests: per-function coverage, edge cases (empty inputs, out-of-range k, no-match defaults, invalid bounds), registration check, integration via `Evaluate` and `EvaluateFormula`