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Fixes the bug reported in #516 where frame.ColumnTypes incorrectly changes to System.Object after filtering rows when the resulting column contains only missing values.

Root Cause

Frame.filterRows, Frame.filterRowValues, and Frame.Where all previously reconstructed the frame from rows (via FrameUtils.fromRowsAndColumnKeys). That function uses a "witness value" to determine the column type — it picks any non-missing value from the column to drive type dispatch:

let someValue =
  nested |> Series.observations |> Seq.tryPick (fun (_, v) ->
    v.TryGetObject(key) |> OptionalValue.asOption)
let someValue = defaultArg someValue (obj())  // ← falls back to obj() when all missing
columnCreator key someValue

When all values in a column are missing after filtering, Seq.tryPick returns None, the fallback becomes obj(), and the resulting column vector has ElementType = typeof(obj). Hence frame.ColumnTypes reports System.Object instead of the original type.

Fix

Replace the row-reconstruction path in filterRows, filterRowValues, and both Where overloads with the same column-wise approach already used by filterRowsBy and realignRows:

1. Apply the predicate to collect the filtered row index.
2. Use IndexBuilder.Reindex to produce a Relocate vector command that maps the new row addresses back to the original ones.
3. Apply VectorHelpers.transformColumn to each column vector — this preserves the generic type parameter 'T of each column regardless of how many values are missing.

Also adds open Deedle.Vectors to FrameExtensions.fs so VectorConstruction.Return is in scope.

Behaviour preserved

• All values (present or missing) are correctly mapped to their new positions.
• Empty filter result still yields an empty frame with the same column keys and types.
• No change in behaviour for columns where at least one value is non-missing.

Test

New regression test Filter rows preserves column types when all column values are missing in tests/Deedle.Tests/Frame.fs verifies that column ElementType remains float after filtering to rows where a column contains only NaN (missing) values.

Test Status

• ✅ dotnet build src/Deedle/Deedle.fsproj -c Release — 0 errors
• ✅ dotnet test tests/Deedle.Tests/Deedle.Tests.fsproj -c Release — 465 / 465 passed

⚠️ CI docs-generation (fsdocs) will fail on this PR because the XML doc issue in master (fixed by PR #597) has not yet been merged. This is an infrastructure failure unrelated to this change — all tests pass.

Closes #516

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