[Repo Assist] perf: HtmlNode.serialize avoids per-indent string alloc; isVoidElement as module-level constant

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Summary

Three focused performance improvements to HtmlNode / HtmlDocument serialization.

Changes

1. isVoidElement as a module-level constant

Before: HtmlNode.ToString() reconstructed a Set<string> of 16 void element names on every call, then wrapped it in a closure.

After: The set (and the isVoidElement predicate) are computed once at module initialisation time, stored in a [<AutoOpen>] module private HtmlNodeHelpers. This eliminates one Set.ofArray + 16-element insertion per ToString() call.

2. newLine uses StringBuilder.Append(char, int) directly

Before:

let newLine plus =
    sb.AppendLine() |> ignore
    String(' ', indentation + plus) |> append   // allocates a temporary string

After:

let newLine plus =
    sb.AppendLine() |> ignore
    sb.Append(' ', indentation + plus) |> ignore  // writes spaces directly, no string alloc

StringBuilder.Append(char, repeatCount) writes the character into the internal buffer directly without creating an intermediate string. Every pretty-printed element that starts a new line previously paid one allocation; now it pays zero.

3. HtmlDocument.ToString() uses a single StringBuilder

Before:

+ (elements |> List.map (fun x -> x.ToString()) |> String.Concat)
```
Each `HtmlNode.ToString()` created its own `StringBuilder`, serialised to it, and returned a `string`. The outer code then built an `FSharpList` of those strings before concatenating.

**After**: A single `StringBuilder` is created; `element.ToString()` is appended directly. The intermediate list allocation is gone.

## Test Status

```
Passed! - Failed: 0, Passed: 2980, Skipped: 0, Total: 2980 (net8.0, -p:NuGetAudit=false)

Pre-existing infrastructure warning (OpenTelemetry.Api 1.15.0 vulnerability via NuGet audit) suppressed with -p:NuGetAudit=false; unrelated to this change, tracked in PR #1762.

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Commit pushed: 7f959f0

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