removed comptime parsing - cron expressions come from config/databases
at runtime, so compile-time validation doesn't serve a real purpose.

added:
- Iterator struct with standard zig ?T pattern
- Field.range() method
- comprehensive doc comments on type system choices

kept:
- StaticBitSet for bounded integer domains
- error unions for explicit error handling
- options struct pattern

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- parse 5-field cron expressions (minute hour day month dow)
- support wildcards, ranges, steps, lists
- month/day name aliases (jan-dec, sun-sat)
- next occurrence calculation
- timestamp matching
- standard day/dow OR logic with wildcard handling

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>