feat: add defaultSelected selection prop

[gpbl]

Adds defaultSelected so apps can initialize uncontrolled selections without relying on selected as initial state. Keeps existing selected behavior intact for v10 compatibility; read-only controlled selections can use selected with a no-op onSelect.

What Changed

• Added defaultSelected to single, multiple, and range selection prop types.
• Initialized uncontrolled selection hooks from defaultSelected, falling back to existing selected behavior when omitted.
• Normalized defaultSelected through the existing timeZone conversion path.
• Updated selection docs, focused tests, and changeset coverage.

[changeset-bot]

🦋 Changeset detected

Latest commit: 00a5855

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
react-day-picker	Minor
@daypicker/react	Minor
@daypicker/buddhist	Minor
@daypicker/ethiopic	Minor
@daypicker/hebrew	Minor
@daypicker/hijri	Minor
@daypicker/persian	Minor

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[rodgobbi]

@gpbl IMO uncontrolled mode is not useful for DayPicker.
The consumers want to have access to the selected dates to use for something else.

Using DayPicker uncontrolled is just a calendar day picker that changes state, but that state is not used for anything else.

[gpbl]

@gpbl IMO uncontrolled mode is not useful for DayPicker. The consumers want to have access to the selected dates to use for something else.

Using DayPicker uncontrolled is just a calendar day picker that changes state, but that state is not used for anything else.

Fair point, and sorry for not providing enough context in this draft PR. The goal is to introduce the defaultSelected prop so that, in a future major release, we can address #2690.

I am still exploring the right solution here, so any feedback is very welcome!

[rodgobbi]

@gpbl I see, I also revisited the discussion in the issue.
Keep in mind the existing issue with undefined representing an empty selection.

What do you think of trying this together with another prop that changes the empty selection from undefined to null?