Perf/code: replace ref cells with mutable in ~30 API functions

Modernises ~30 public API functions to use 'mutable' local variables
instead of heap-allocated 'ref' cells (! and := operators). This
reduces GC pressure in hot paths and aligns the code with idiomatic
modern F#.

Affected functions: tryLast, tryFirst, tryItem, compareWithAsync,
reduceAsync, scanAsync, pairwise, windowed, pickAsync, tryPickAsync,
tryFindIndex, tryFindIndexAsync, threadStateAsync, zipWithAsync,
zipWithAsyncParallel, zipWithAsync3, allPairs, takeWhileAsync,
takeUntilSignal, skipWhileAsync, skipWhileInclusiveAsync,
skipUntilSignal, tryTail, splitAt, toArrayAsync, concatSeq,
interleaveChoice, chunkBySize, chunkByAsync, mergeChoiceEnum,
distinctUntilChangedWithAsync, emitEnumerator, removeAt, updateAt,
insertAt, ofObservableBuffered, Disposables.Dispose.

All 372 tests pass.

Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -1,5 +1,6 @@
 ### 4.11.0
 
+* Code/Performance: Modernised ~30 API functions to use `mutable` local variables instead of `ref` cells (`!`/`:=` operators). Affected: `tryLast`, `tryFirst`, `tryItem`, `compareWithAsync`, `reduceAsync`, `scanAsync`, `pairwise`, `windowed`, `pickAsync`, `tryPickAsync`, `tryFindIndex`, `tryFindIndexAsync`, `threadStateAsync`, `zipWithAsync`, `zipWithAsyncParallel`, `zipWithAsync3`, `allPairs`, `takeWhileAsync`, `takeUntilSignal`, `skipWhileAsync`, `skipWhileInclusiveAsync`, `skipUntilSignal`, `tryTail`, `splitAt`, `toArrayAsync`, `concatSeq`, `interleaveChoice`, `chunkBySize`, `chunkByAsync`, `mergeChoiceEnum`, `distinctUntilChangedWithAsync`, `emitEnumerator`, `removeAt`, `updateAt`, `insertAt`. This eliminates heap-allocated `ref`-cell objects for these variables, reducing GC pressure in hot paths, and modernises the code style to idiomatic F#.
 * Performance: `mapiAsync` — replaced `asyncSeq`-builder + `collect` implementation with a direct optimised enumerator (`OptimizedMapiAsyncEnumerator`), eliminating `collect` overhead and bringing per-element cost in line with `mapAsync`. Benchmarks added in `AsyncSeqMapiBenchmarks`.
 * Design parity with FSharp.Control.TaskSeq (#277, batch 2):
   * Added `AsyncSeq.tryTail` — returns `None` if the sequence is empty; otherwise returns `Some` of the tail. Safe counterpart to `tail`. Mirrors `TaskSeq.tryTail`.