[Repo Assist] feat: add TaskSeq.groupBy, countBy, and partition (195 tests)

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Summary

Adds six new combinators to TaskSeq, completing a common set of aggregation and splitting operations that mirror List, Array, and Seq equivalents:

Function	Description	Return type
TaskSeq.groupBy	Groups elements by sync key projection	Task<('Key * 'T[])[]>
TaskSeq.groupByAsync	Groups elements by async key projection	Task<('Key * 'T[])[]>
TaskSeq.countBy	Counts elements per key	Task<('Key * int)[]>
TaskSeq.countByAsync	Counts elements per key (async)	Task<('Key * int)[]>
TaskSeq.partition	Splits by predicate into two arrays	Task<'T[] * 'T[]>
TaskSeq.partitionAsync	Splits by async predicate into two arrays	Task<'T[] * 'T[]>

Design notes

• groupBy / countBy return Task<...> (not TaskSeq<...>) because they must consume the entire sequence before forming groups — the same reason List.groupBy returns a list rather than a lazy sequence.
• Both return results in first-occurrence key order, matching Seq.groupBy / Seq.countBy behaviour.
• Keys use HashIdentity.Structural for structural equality, consistent with except/distinctBy.
• A new [(Struct)] ProjectorAction DU was added alongside the existing FolderAction, PredicateAction, etc. to share one internal implementation for sync/async projections.
• partition reuses the existing PredicateAction DU.

Examples

// Group integers by parity
let! groups = taskSeq { 1..10 } |> TaskSeq.groupBy (fun x -> x % 2 = 0)
// [| (false, [|1;3;5;7;9|]); (true, [|2;4;6;8;10|]) |]

// Count words by first character
let! counts = words |> TaskSeq.countByAsync (fun w -> task { return w[0] })

// Split items
let! evens, odds = taskSeq { 1..10 } |> TaskSeq.partition (fun x -> x % 2 = 0)
// evens = [|2;4;6;8;10|], odds = [|1;3;5;7;9|]

Files changed

• src/FSharp.Control.TaskSeq/TaskSeqInternal.fs — ProjectorAction DU + groupBy, countBy, partition implementations
• src/FSharp.Control.TaskSeq/TaskSeq.fs — 6 new static members
• src/FSharp.Control.TaskSeq/TaskSeq.fsi — full XML doc signatures with when 'Key: equality constraints
• src/FSharp.Control.TaskSeq.Test/TaskSeq.GroupBy.Tests.fs — 195 tests (null checks, empty, immutable, side-effect variants for all 6 functions)
• src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj — adds new test file
• release-notes.txt — updated for 0.5.0

Test Status

✅ Build succeeded — 0 warnings, 0 errors
✅ All 4045 existing tests pass (release configuration)
✅ 195 new GroupBy/CountBy/Partition tests pass
✅ Formatting clean (dotnet fantomas . --check passes)

ref #289

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