[Repo Assist] perf: use while! in iter/fold/reduce/mapFold/tryLast/Drop/Truncate

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Summary

This PR replaces the manual go-flag + initial-MoveNext pre-advance pattern with idiomatic while! in six functions: iter, fold, reduce, mapFold, tryLast, and the Drop/Truncate cases of skipOrTake.

The pattern being replaced

Before (in iter, fold, reduce, mapFold, tryLast):

let mutable go = true
let! step = e.MoveNextAsync()
go <- step

while go do
    // body
    let! step = e.MoveNextAsync()
    go <- step

After:

while! e.MoveNextAsync() do
    // body

This matches the pattern already established by sum, sumBy, average, averageBy, lengthBy, lengthBeforeMax, and others.

Changes

Function	Before	After
iter	manual go flag, initial pre-advance	while! e.MoveNextAsync() per branch
fold	manual go flag, initial pre-advance	while! e.MoveNextAsync() per branch
reduce	manual go flag, second pre-advance after first-element check	while! e.MoveNextAsync() per branch
mapFold	manual go flag, initial pre-advance	while! e.MoveNextAsync() per branch
tryLast	manual go flag, initial pre-advance	while! e.MoveNextAsync()
skipOrTake/Drop	pos counter with inner if pos < count	i < count as loop guard, if cont for rest
skipOrTake/Truncate	pos counter with inner if pos < count	yielded < count as loop guard

Benefits

• Removes one mutable bool per function (the go flag)
• Removes the initial pre-advance let! step = e.MoveNextAsync(); go <- step before the loop
• Hoists the match action/folder/... dispatch outside the while loop in iter and fold (it was already outside in the original, but the new form makes it clearer that each branch owns its own tight loop)
• Unifies style across the module — new contributors see one consistent idiom

Scope

All changes are purely internal — no public API or behaviour change.

Trade-offs

• None. This is a straightforward pattern replacement with identical semantics, verified by the full test suite.

Test Status

✅ Build: 0 warnings, 0 errors (Release)
✅ Fantomas: formatting check passes
✅ Tests: 5093 passed, 2 skipped, 0 failed

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