let traits decide how the two day fields combine

[mariusbancila]

0 0 0 1 1 1 - the 1st of January, and Mondays in January — returned 2024-01-01, because croncpp required a date to match both day fields. Every cron dialect disagrees with that, and they disagree with each other too: POSIX matches a date against either field when both are restricted, while Quartz refuses the expression outright and wants ? in one of them.

Rather than pick a winner, each traits type now states the rule of the dialect it describes, via a new CRON_DAY_FIELD_RULE:

┌────────────────────────────────────────┬──────────────────────────────────────────┐
│                 Traits                 │                   Rule                   │
├────────────────────────────────────────┼──────────────────────────────────────────┤
│ cron_standard_traits                   │ either, as in POSIX cron                 │
├────────────────────────────────────────┼──────────────────────────────────────────┤
│ cron_quartz_traits, cron_oracle_traits │ reject; one field has to be ?            │
├────────────────────────────────────────┼──────────────────────────────────────────┤
│ traits declaring nothing               │ intersect, croncpp's behaviour until now │
└────────────────────────────────────────┴──────────────────────────────────────────┘

So the reported expression now resolves to 2021-01-01 under the standard traits, and is rejected with a clear message under the other two. Traits written before this change keep intersecting, so custom traits are unaffected — same opt-in shape as the year field.

A field counts as restricted unless it is literally * or ?. That distinction is recorded at parse time and included in operator==, since 0 0 0 1-31 1 1 and 0 0 0 * 1 1 set identical bits but no longer mean the same thing.

This is a behaviour change for expressions restricting both day fields under the standard traits, and a new error under quartz and oracle. Nothing in the existing suite moved - none of it restricts both fields, but it belongs in the release notes.

New test_day_fields.cpp.

Fixes #12.