[Repo Assist] perf: use HashSet(char) and String.exists for adorner detection in TextConversions

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What and Why

Two small improvements to TextConversions.RemoveAdorners, which is called on every cell value during CSV/JSON/XML parsing for AsInteger, AsDecimal, and AsInteger64.

1. HashSet<char> instead of F# Set<char> for the private adorner set

-static member val private DefaultRemovableAdornerCharacters =
-    Set.union TextConversions.DefaultNonCurrencyAdorners TextConversions.DefaultCurrencyAdorners
+static member val private DefaultRemovableAdornerCharacters =
+    // HashSet<char> for O(1) membership testing (vs O(log n) for F# Set)
+    HashSet<char>(
+        Set.union TextConversions.DefaultNonCurrencyAdorners TextConversions.DefaultCurrencyAdorners
+    )

The set has ~26 characters. F# Set.Contains is O(log n) (balanced BST). HashSet<char>.Contains is O(1). The public members DefaultNonCurrencyAdorners and DefaultCurrencyAdorners remain Set<char> — no API change.

2. String.exists with early exit instead of a manual loop

-let mutable hasAdorners = false
-for i = 0 to value.Length - 1 do
-    if TextConversions.DefaultRemovableAdornerCharacters.Contains(value.[i]) then
-        hasAdorners <- true
+// Fast path: short-circuit on first adorner found (String.exists stops early)
+let hasAdorners =
+    value |> String.exists (fun c -> TextConversions.DefaultRemovableAdornerCharacters.Contains(c))

The previous loop always iterated to the end of the string even after finding an adorner. String.exists short-circuits on the first match. For values like "$1,234", the adorner $ is at index 0, so the remaining 5 characters are unnecessary work.

Test Status

• ✅ dotnet test tests/FSharp.Data.Core.Tests/ — 2896 passed, 0 failed

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