enhancement: add job priority/ordering support

[CybotTM]

Description

When multiple jobs are scheduled for the same time, there's no way to control execution order.

Current Behavior

Jobs scheduled for the same time execute in arbitrary order based on heap implementation.

Proposed Enhancement

// Priority option (higher = runs first)
c.AddFunc("@daily", highPriorityJob, cron.WithPriority(10))
c.AddFunc("@daily", lowPriorityJob, cron.WithPriority(1))

// Or dependency-based
c.AddFunc("@daily", jobB, cron.After(jobAID))  // See #192

// Or explicit ordering
c.AddFunc("@daily", job1, cron.WithOrder(1))
c.AddFunc("@daily", job2, cron.WithOrder(2))

Use Cases

• Run cleanup jobs before main jobs
• Ensure resource preparation before consumers
• Predictable execution order for testing
• Priority scheduling under load

Severity

🟢 Low - Feature enhancement

Found By

Pre-1.0 release code review with Zen/Gemini