[Repo Assist] feat: add TaskSeq.mapFold and TaskSeq.mapFoldAsync (56 tests)

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Summary

Adds TaskSeq.mapFold and TaskSeq.mapFoldAsync, which combine a map and a fold in a single pass over a task sequence — mirroring List.mapFold, Array.mapFold, and Seq.mapFold from FSharp.Core.

Signature

TaskSeq.mapFold :
    mapping: ('State -> 'T -> 'Result * 'State)
    -> state: 'State
    -> source: TaskSeq<'T>
    -> Task<'Result[] * 'State>

TaskSeq.mapFoldAsync :
    mapping: ('State -> 'T -> #Task<'Result * 'State>)
    -> state: 'State
    -> source: TaskSeq<'T>
    -> Task<'Result[] * 'State>

The mapping function receives the current state and element, returns a mapped result and a new state. All elements are consumed eagerly (like List.mapFold), producing an array of results and the final state.

Changes

• TaskSeqInternal.fs: new MapFolderAction struct DU; mapFold internal implementation using a ResizeArray collector
• TaskSeq.fs: two delegating members (mapFold, mapFoldAsync)
• TaskSeq.fsi: full XML doc-comment signatures for both members
• TaskSeq.MapFold.Tests.fs: 56 tests across EmptySeq, Functionality, and SideEffects modules — including parity test against List.mapFold
• release-notes.txt: updated for 0.5.0

Trade-offs

• Result is 'Result[] (not TaskSeq<'Result>) because the final state is only known after the full sequence is consumed. This matches FSharp.Core's eager semantics for mapFold.
• A lazy variant yielding (TaskSeq<'Result> * Task<'State>) is theoretically possible but would require buffering or two-pass consumption, and would diverge from idiomatic F# usage.

Test Status

✅ Build succeeded (0 warnings, 0 errors)
✅ All 56 new mapFold/mapFoldAsync tests pass
✅ Full test suite: 3906 passed, 2 skipped (infra), 0 failed
✅ Fantomas format check passes

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