[Repo Assist] perf: HtmlCharRefs entity lookup uses Dictionary for O(1) instead of Map O(log n)

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Summary

HtmlCharRefs.refs previously used an F# Map<string, string>, which gives O(log n) lookups over its ~2230 entries (≈ 11 string comparisons per lookup). This PR replaces it with a Dictionary<string, string> initialised with StringComparer.Ordinal, giving O(1) lookups.

A secondary micro-optimisation avoids allocating a new char[] on every call to TrimEnd inside the (|Number|Lookup|) active pattern by sharing a private static semiColonChars array.

Root Cause

The entity table was populated with |> Map.ofArray (F# persistent balanced-BST map). For HTML documents with many character references (e.g. &, <,   etc.) every entity name is resolved via a tree traversal. A Dictionary with ordinal key comparison gives the same correct result with constant-time lookup.

Changes

• src/FSharp.Data.Html.Core/HtmlCharRefs.fs
  • Split the ~2230-entry array into a private binding refsEntries
  • Build refs : Dictionary<string, string> from that array with capacity hint and StringComparer.Ordinal
  • Add private semiColonChars = [| ';' |] (reused by TrimEnd)
  • Replace defaultArg (refs.TryFind ref) ref with refs.TryGetValue pattern match
• RELEASE_NOTES.md — new 8.1.10 entry
• src/AssemblyInfo*.fs — regenerated with version 8.1.10.0

Test Status

• ✅ Build succeeded (0 errors, 34 pre-existing warnings)
• ✅ dotnet test tests/FSharp.Data.Core.Tests --configuration Release — 2957 passed, 0 failed
• ✅ dotnet fantomas --check src/FSharp.Data.Html.Core/HtmlCharRefs.fs — formatting compliant

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