feat: Add support for negative years (BC dates) in Year field

[CybotTM]

Summary

Enable support for negative years (BC dates) in the Year field for research, historical simulation, and experimental use cases.

Background

With PR #253, the Year field now uses sparse map[int]struct{} storage with expanded bounds (1 CE to MaxInt32). However, negative years (BC dates) are not yet supported.

Go's time.Time fully supports negative years:

time.Date(-44, 3, 15, 0, 0, 0, 0, time.UTC) // March 15, 44 BC (Ides of March!)

Challenges

Syntax Problem

The standard cron syntax uses - as a range separator, making negative years ambiguous:

• "-44" would be parsed as empty start - to 44 (invalid range)

Proposed Solutions

1. Add ~ prefix for negative years

   • ~44 = year 44 BC (-44)
   • ~100-~44 = range from 100 BC to 44 BC
   • Example: 0 0 15 3 * ~44 = midnight on March 15, 44 BC

2. Add WithYearBounds(min, max int) method

   • Allows users to configure custom year bounds per parser
   • Example: parser.WithYearBounds(-10000, 10000) for 10000 BC to 10000 AD

Both solutions are needed: the syntax (#1) enables expressing negative years, and the bounds method (#2) enables validating them.

Proposed Implementation

// New syntax: ~ prefix for negative years
parser := NewParser(Minute | Hour | Dom | Month | Dow | Year).
    WithYearBounds(-10000, 10000)

// Ides of March, 44 BC - midnight on March 15
sched, _ := parser.Parse("0 0 15 3 * ~44")

// Range from 100 BC to 44 BC
sched, _ := parser.Parse("0 0 1 1 * ~100-~44")

Tasks

• Update getYearRange() to parse ~ prefix as negative
• Add WithYearBounds(min, max int) Parser method
• Update YearBase constant or make it configurable
• Add tests for negative year parsing
• Add tests for year bounds configuration
• Update documentation

Use Cases

While primarily for experimentation and research, potential use cases include:

• Historical event simulations
• Astronomy/calendar calculations
• Educational tools
• Game development with historical settings