Improve DWIM on straight datetime days

## Problem

This is not quite DWIM.

```python
>>> from aika import TimeIntervalParser
>>> TimeIntervalParser().parse("2025-02-25").githubformat()
'2025-02-25..2025-03-01'
```

## Pitch

This would be better.

```python
>>> from aika import TimeIntervalParser
>>> TimeIntervalParser().parse("2025-02-25").githubformat()
'2025-02-25..2025-02-25'
```
