feat!: Validate step size against range in cron expressions

[CybotTM]

Summary

This change validates that step sizes in cron expressions are meaningful for the given range, preventing silent misbehavior.

Problem

In the original robfig/cron, expressions like */60 in the minute field (range 0-59) were accepted but produced unexpected results:

// Before: Accepted but confusing
_, err := c.AddFunc("*/60 * * * *", myFunc)
// err is nil, but job only runs at minute 0 (not every 60 minutes!)

The step of 60 is larger than the range size (60 values: 0-59), making it semantically meaningless.

Solution

Parser now validates that:

• Step size must be positive (> 0)
• Step size must be less than the range size (for steps > 1)

Error message: step (60) must be less than range size (60): */60

Examples of Invalid Expressions

Expression	Field	Range	Step	Error
*/60 * * * *	minute	0-59 (60 values)	60	step >= range size
*/25 * * * *	hour field	0-23 (24 values)	25	step >= range size
*/32 * * *	day of month	1-31 (31 values)	32	step >= range size
*/0 * * * *	any	any	0	step must be positive

Valid Expressions

• */30 * * * * - Every 30 minutes (step 30 < range 60) ✓
• 0 */2 * * * - Every 2 hours (step 2 < range 24) ✓
• 0 0 */7 * * - Every 7 days (step 7 < range 31) ✓

Migration

Users with invalid step expressions will receive a parse error. Review and fix expressions:

• */60 * * * * → 0 * * * * (every hour at minute 0)
• */90 * * * * → 0 */2 * * * (every 2 hours)

Stability Impact

• Severity: Low
• Type: Input validation improvement
• Prevents confusing behavior where jobs don't run as expected

Related

• Part of v0.5.0 release
• Implemented in PR feat: modernize fork with panic fixes, DST handling, and CI #1
• Addresses upstream issue ParseStandard not validating step range robfig/cron#543

[CybotTM]

Documented and released in v0.5.0. This issue was created retroactively for release notes documentation.