[repo-assist] Add Frame.Between/After/Before/StartAt/EndAt row-range slicing

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Summary

Adds Frame.Between / Frame.After / Frame.Before / Frame.StartAt / Frame.EndAt members (and matching Frame module functions), mirroring the row-range slicing API that already exists on Series (Series.Between/After/Before/StartAt/EndAt).

Rationale

Slicing a frame's rows by a key range (e.g. a date range) is a common time-series operation, but Frame had no direct equivalent to the Series range members — users had to write:

df.Rows |> Series.between lo hi |> Frame.ofRows

Now this is:

df.Between(lo, hi)

Implementation reuses the existing Frame.GetSubrange helper (already used internally for Frame's slice operator), so the new members are thin, low-risk wrappers — exactly the same pattern already used for Frame.head/Frame.tail/Frame.chunk in earlier releases (see RELEASE_NOTES.md).

Trade-offs

• None identified — purely additive API surface, no changes to existing behavior.

Test Status

• ✅ dotnet build src/Deedle/Deedle.fsproj -c Release — succeeded, no new warnings.
• ✅ dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release — 722/722 passed (720 existing + 2 new test functions covering both the members and the module-function wrappers).

Updated RELEASE_NOTES.md under the unreleased 8.1.0 section.

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