[repo-assist] perf: HtmlParser reuses CurrentTag and Content StringBuilders across tokens

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Summary

The HTML parser's HtmlState type previously allocated a new { Contents = StringBuilder() } (a new CharList record and a new StringBuilder) on every call to:

• EmitSelfClosingTag() — reset CurrentTag
• EmitTag() — reset CurrentTag
• EmitToAttributeValue() — reset Content
• Emit() — reset Content

For a typical HTML document with hundreds of elements, this means hundreds of short-lived StringBuilder objects, all discarded immediately after .Clear() would have sufficed.

Fix

Replace x.CurrentTag <- { Contents = StringBuilder() } and x.Content <- { Contents = StringBuilder() } with x.CurrentTag.Clear() and x.Content.Clear() respectively.

The CharList.Clear() method already existed for exactly this purpose (line 90).

Benefit

• Eliminates 2 heap allocations per HTML element (one CharList record + one StringBuilder) for both CurrentTag and Content resets — roughly 4 allocations per opening/closing tag pair.
• For a 1000-element HTML page this removes ~4000 short-lived allocations from the parser hot path.
• No change to observable behaviour.

Trade-offs

None — this is a pure internal refactor. The CharList struct already reuses its StringBuilder safely via .Clear().

Test Status

All 2284 HTML-related tests pass on Linux/net8.0 (FSharp.Data.Core.Tests, HTML filter).

Closes #none (standalone performance improvement)

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