🤖 Repo Assist — automated PR implementing the feature requested in issue #372.

Closes #372

What this adds

C# extension methods (Rank, RankWith, Ntile, RankRowsBy) are provided in the corresponding *Extensions.fs files.

Usage examples

// Dense rank
let ages = series ["alice" => 30.0; "bob" => 25.0; "carol" => 30.0; "dave" => 20.0]
ages |> Series.rank
// dave→1, bob→2, alice→3, carol→3

// Ntile into 3 equal buckets
ages |> Series.ntile 3
// dave→0, bob→0, alice→1, carol→2  (or similar, by sorted position)

// Add a rank column to a frame
let df = frame [ "age" => ages ]
df.AddColumn("rank", df |> Frame.rankRowsBy "age")

Design notes

• Missing values are propagated as missing in the output (not ranked).
• Series.rank uses Map<'V, int> for O(n log n) lookup.
• Series.rankWith walks a sorted array; lookup is O(n) per element (acceptable for typical series sizes; the common numeric use-case should use Series.rank).
• Series.ntile sorts by value via Seq.sortBy snd on observations, so only present values are bucketed.

Test status

All 616 existing tests pass. Eight new tests were added:

• Series.fs: rank basic, rank with ties, rank with missing values, rank with custom comparator, ntile equal buckets, ntile uneven buckets
• Frame.fs: rank rows by column, rank and add as column

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