Add ChronosInterval class

[dereuromark]

Summary

This PR adds a ChronosInterval class that wraps DateInterval using the decorator pattern, as suggested by @markstory in #444.

Features

• ISO 8601 Duration Formatting: __toString() returns proper ISO 8601 format (e.g., P1Y2M3DT4H5M6S)
• Factory Methods:
  • create(string $spec) - Create from ISO 8601 spec
  • createFromValues(...) - Create from individual components
  • instance(DateInterval $interval) - Wrap an existing DateInterval
• Native Interop: toNative() returns the underlying DateInterval for compatibility
• Convenience Methods:
  • totalSeconds() - Approximate total seconds
  • totalDays() - Total days (exact if from diff())
  • isNegative() - Check if interval is inverted
  • isZero() - Check if interval has no duration
• Property Access: Proxies y, m, d, h, i, s, f, invert, days

Example Usage

// Create from spec
$interval = ChronosInterval::create('P1Y2M3D');
echo $interval; // "P1Y2M3D"

// Create from values
$interval = ChronosInterval::createFromValues(hours: 2, minutes: 30);
echo $interval; // "PT2H30M"

// Wrap existing DateInterval
$diff = $date1->diff($date2);
$interval = ChronosInterval::instance($diff);

// Use with native code
someFunctionExpectingDateInterval($interval->toNative());

Discussion Points

This is a draft PR to gather feedback on the API design:

1. Should we add arithmetic methods (add(), sub())?
2. Should ChronosInterval be returned from Chronos::diff() instead of DateInterval?
3. Any other methods that would be useful?

Related to #444

[LordSimal]

1. Should we add arithmetic methods (add(), sub())?
   Yes, I can definitely see it being usefull.

2. Should ChronosInterval be returned from Chronos::diff() instead of DateInterval?
   For the next major, I'd say yes. Especially if we implement createFromDateString(string $datetime) as well and just map it to the create() method.

3. Any other methods that would be useful?
   Other than createFromDateString I can see methods like ->next(Chronos::TUESDAY) like we have in the Chronos class would be cool to adjust an interval to a desired range.

[markstory]

Looks good to me. The decorator based design avoids all the problems we've had with extending datetime classes in the past.

[markstory]

Should we add arithmetic methods (add(), sub())?

I think these are useful for combining intervals together.

Should ChronosInterval be returned from Chronos::diff() instead of DateInterval?

Wouldn't this be a breaking change? I don't think we can make that switch outside of a major release.

Any other methods that would be useful?

Perhaps a method to convert the interval in to a strottime() compatible expression?

[dereuromark]

@LordSimal Could you clarify what you meant by ->next(Chronos::TUESDAY) style methods for intervals?

In Chronos, next() works from a specific date reference point. But intervals don't have an origin date, so I'm not sure how this would work semantically.

Some possible interpretations:

• Round interval UP to next full unit? (P1Y2M15D -> P1Y3M)
• Align/snap to calendar boundaries? (10 days -> 2 weeks)
• Something else?

Would appreciate an example of the use case you had in mind.

[dereuromark]

Docs added.