[Repo Assist] perf+feat: optimize exists/contains; add TaskSeq.distinct/distinctBy/distinctByAsync

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Summary

This PR bundles two improvements from the selected runs (Task 8: Performance, Task 5: Coding Improvements):

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Performance: optimize exists / existsAsync / contains

Before: all three functions went through tryFind, which boxes the found element into a heap-allocated Some 'T before the caller immediately discards it via Option.isSome. contains additionally created a (=) value partial-application closure.

After: a dedicated exists function (mirroring the existing forall implementation) returns bool directly, with an early-exit loop that never touches Option. contains uses its own tight loop with an inline equality check.

This eliminates one heap allocation per successful call and avoids a closure allocation for contains.

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Coding improvement: TaskSeq.distinct / distinctBy / distinctByAsync

Adds three new combinators that complement the existing distinctUntilChanged, except, and exceptOfSeq:

Function	Description
TaskSeq.distinct	Removes all duplicates (keeps first occurrence), equality on the element
TaskSeq.distinctBy projection	Removes elements whose projected key has already been seen
TaskSeq.distinctByAsync projection	Async variant of distinctBy

All three use a HashSet and iterate the source once. Like except, they are not suitable for infinite sequences.

Difference from distinctUntilChanged: distinctUntilChanged only removes consecutive duplicates (like uniq). distinct removes all duplicates regardless of position (like sort | uniq -u).

taskSeq { yield! [1; 2; 1; 3; 2] }
|> TaskSeq.distinct
// → [1; 2; 3]

taskSeq { yield! [1; 2; 1; 3; 2] }
|> TaskSeq.distinctUntilChanged
// → [1; 2; 1; 3; 2]  (only consecutive removed)

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Files changed

• TaskSeqInternal.fs — new exists, contains, distinct, distinctBy, distinctByAsync implementations
• TaskSeq.fsi — public API signatures with full XML doc comments
• TaskSeq.fs — wire-up
• TaskSeq.Distinct.Tests.fs (new) — 75 tests
• FSharp.Control.TaskSeq.Test.fsproj — test file registration
• release-notes.txt — updated

Test Status

Build: ✅ succeeded (Release)
Tests: ✅ 3893 passed, 0 failed, 2 skipped (pre-existing infrastructure skips)

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