Build offline XForm constraint validation library

Create a TypeScript library that evaluates XForm constraint expressions locally, without network access, so field workers catch data entry errors immediately.

validateConstraint(
  expression: string,
  value: unknown,
  formContext: Record<string, unknown>  // other field values in scope
): { valid: boolean, message?: string }

Implement evaluation for these XPath functions commonly used in ODK forms:

• string-length(expr) — returns character count
• regex(expr, pattern) — returns boolean
• number(expr) — casts to number
• if(condition, then, else) — ternary
• coalesce(a, b) — returns first non-empty value
• selected(field, value) — true if multi-select field contains value
• count-selected(field) — count of selected values

Comparison operators to support: =, !=, <, <=, >, >=, and, or, not().

Parse expressions with a simple recursive descent parser. No external parser libraries.

Write a test suite with 20 XForm constraint examples: phone number regex, date range check, required-if-other-selected, numeric bounds, string length limits.