🤖 This is an automated fix from Repo Assist.

Closes #95

Root cause

Series.FormatStrings, Frame.FormatStrings, and Series.ToString() all format index keys using .ToString() directly. For DateTime values, this always includes the time component — even when it is 00:00:00 — producing noisy output like 1/1/2023 12:00:00 AM for every row of a day-granularity time series.

The CSV exporter in FrameUtils.fs already applied a per-key rule: if dt.TimeOfDay = TimeSpan.Zero format as date-only ("d"). The FSI prettyprint code was missing this behaviour.

Fix

Added a Formatting.formatKey helper to Common.fs:

let formatKey (key:obj) =
  match key with
  | :? DateTime as dt when dt.TimeOfDay = TimeSpan.Zero ->
      dt.ToString("d", Globalization.CultureInfo.CurrentCulture)
  | _ -> key.ToString()
```

Then wired it in to the three call sites:

- `Frame.fs` → `getLevel` (row/column header rendering)
- `Series.fs` → `getLevel` (table rendering) and `Series.ToString()` (inline FSI output)

## Before / After

**Before:**
```
1/1/2023 12:00:00 AM -> 1.5
1/2/2023 12:00:00 AM -> 2.1
1/3/2023 12:00:00 AM -> 3.7
```

**After:**
```
1/1/2023 -> 1.5
1/2/2023 -> 2.1
1/3/2023 -> 3.7

DateTime keys with a non-zero time component are unaffected.

Trade-offs

• Per-key decision (not "all-or-nothing"): a series with mixed date-only and datetime keys will have narrower formatting for the date-only rows. In practice, mixed series are rare.
• Uses CultureInfo.CurrentCulture for locale-appropriate formatting, matching the CSV exporter.
• Zero extra allocations for non-DateTime keys.

Test Status

• ✅ 4 new regression tests added to tests/Deedle.Tests/Formatting.fs
• ✅ All 592 tests pass (dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release)
• ✅ Build succeeded with 0 errors

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