[Repo Assist] Precompile markdown parser regexes to avoid per-call construction

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Summary

Three regex patterns in the Markdown parser were constructed or compiled on every call to their containing active patterns, making them unnecessarily expensive hot paths.

Before

MarkdownInlineParser.fs — Punctuation pattern (called for every character during inline parsing):

let (|Punctuation|_|) input =
    match input with
    | EscapedChar _ -> None
    | _ ->
        let re = """^[!"#$%&'()*+,...huge ~1.5 KB string...]"""
        let match' = Regex.Match(Array.ofList input |> String, re)  // entire remaining input converted each call
        ...

MarkdownInlineParser.fs — HtmlEntity pattern and MarkdownBlockParser.fs — BlockquoteStart: similar per-call string construction and Regex.Match calls.

After

• All three regex patterns are extracted to private module-level let bindings and compiled with RegexOptions.Compiled, so the NFA is built once at startup.
• Punctuation: passes only the first 2 chars of remaining input (sufficient for all cases: 1 char for BMP punctuation, 2 chars for surrogate-pair cases). Avoids converting potentially thousands of characters on every call.
• HtmlEntity: passes only the first 34 chars (the maximum possible entity length: & + 32 name/digit chars + ;).
• BlockquoteStart: removes per-call string concatenation to build the pattern.

Impact

The Punctuation active pattern is in the inner loop of parseChars, which processes every character in the inline content. For documents with long prose sections this is called thousands of times per parse. Precompiling the regex and shortening the input slice should meaningfully reduce allocation and CPU time in the markdown parser hot path.

Test Status

✅ Build: dotnet build FSharp.Formatting.sln --configuration Release — succeeded (0 errors)
✅ Tests: dotnet test tests/FSharp.Markdown.Tests — 257/257 passed
✅ Formatting: dotnet fantomas — no changes needed after manual format

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