Phase 1 — expression introspection (DX / tooling).

• cron.validateDetailed(expression) → { valid, fields?, errors }. Unlike validate() (boolean), it collects every problem with { field, value, message }, and returns the decomposed fields when valid.
• cron.parse(expression) → the decomposed fields ({ second, minute, hour, dayOfMonth, month, dayOfWeek }), or throws with a useful message (which field, which value) for the first problem.

Both reuse the existing per-field validators — validate() (boolean) is untouched, so no behavior change there. Handles 5/6-field forms, the L token in day-of-month, illegal characters, wrong field count, and non-string input.

cron.validateDetailed('0 30 9 * * 1-5');
// { valid: true, fields: { ..., dayOfWeek: [1,2,3,4,5] }, errors: [] }

cron.validateDetailed('99 * * * * *');
// { valid: false, errors: [{ field: 'second', value: '99', message: '99 is a invalid expression for second' }] }

cron.parse('0 30 9 * * *'); // { second:[0], minute:[30], hour:[9], ... }

New exported types: ParsedFields, DetailedValidation, CronFieldError. Full suite green (327 tests).