Sa.Schedule: move cron parsing out of Engine into dedicated Cron namespace

[dundich]

Refactor the cron scheduling pieces so the parser and timing live in Sa.Schedule.Cron instead of Sa.Schedule.Engine:

• Move CronTiming from Engine to Cron/, splitting field parsing into CronFieldParser + CronField, and adding a dedicated CronParseException.
• Simplify IJobTiming.GetNextOccurrence to a single argument (drop the unused IJobContext parameter); update JobController, tests accordingly.
• Add WithCron on JobProperties and a CronTimingAdapter bridge so the builder wiring keeps working against IJobTiming.
• Drop the now-redundant Engine.JobTiming, add Settings.JobTiming.
• Add detailed EN/RU cron docs under Cron/ and link them from the READMEs.
• Update tests: single-arg calls, empty-expression throws CronParseException, and relaxed invalid-range assertions.