[Repo Assist] perf: replace ref cells with mutable in ofSeq, tryWith, tryFinally enumerators

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🤖 This is an automated pull request from Repo Assist.

Summary

Eliminates unnecessary heap allocations in three fundamental async sequence enumerators by replacing ref cell state fields with mutable locals.

Problem

Three enumerator implementations — ofSeq, tryWith, and tryFinally — each allocated a Ref<T> wrapper object on every call to store the enumerator's state machine variable:

// Before (allocates a Ref<MapState> on the heap)
let state = ref (MapState.NotStarted inp)
...
state.Value <- MapState.HaveEnumerator e
match state.Value with

tryWith also allocated two short-lived Ref<Choice<_,_>> objects per invocation to bridge synchronous try...with logic inside the async { } block.

Fix

Replace ref with mutable, consistent with the pattern already used by collectSeq and takeWhileInclusive (introduced in 4.11.0 / 4.12.0):

// After (mutable becomes a field in the generated class — no separate Ref<T> allocation)
let mutable state = MapState.NotStarted inp
...
state <- MapState.HaveEnumerator e
match state with

In F# object expressions, let mutable values captured by member methods are promoted to fields of the compiler-generated class, so no extra allocation is needed. The Ref<T> wrapper is entirely eliminated.

Impact

• ofSeq: Called any time a seq<'T> is wrapped as an async sequence. One Ref<MapState> allocation eliminated per enumeration.
• tryWith: Used by the asyncSeq { try ... with ... } CE builder. One Ref<TryWithState> + two Ref<Choice> allocations eliminated per enumerator.
• tryFinally: Used by asyncSeq { try ... finally ... } and use bindings. One Ref<TryFinallyState> allocation eliminated per enumerator.

Trade-offs

No behaviour change; purely a GC pressure reduction. The F# compiler generates equivalent IL — the difference is that the state now lives directly as a field of the enumerator class instead of as a field pointing to a separately allocated Ref<T> object.

Test Status

✅ Build: succeeded (0 errors, pre-existing warnings only)
✅ Tests: 422/422 passed (418 existing + 4 new tests for ofSeq, tryFinally resource disposal, and tryWith handler yield)

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