[Repo Assist] perf: StringBuilder in CSS readString; AsSpan in JsonStringEncodeTo on .NET 6+

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Summary

Two small performance improvements eliminating unnecessary allocations in hot paths:

1. HtmlCssSelectors.fs — readString uses StringBuilder

The recursive readString function was accumulating chars by string concatenation (acc + c.ToString()), allocating a new string at every step — O(n²) total allocations for a CSS selector token of length n.

Fix: replaced the string accumulator with a System.Text.StringBuilder, with a thin non-recursive readString wrapper keeping all 13 call sites unchanged.

2. JsonValue.fs — JsonStringEncodeTo uses AsSpan on .NET 6+

JsonStringEncodeTo was using value.Substring(start, len) to flush runs of unescaped characters to a TextWriter, allocating a temporary string. On .NET 6+ (net8.0 target), TextWriter.Write(ReadOnlySpan<char>) is available and avoids this allocation entirely. The netstandard2.0 path keeps Substring.

Trade-offs

• Both changes are pure performance improvements with no semantic change.
• The #if NETSTANDARD2_0 / #else guard adds a small amount of conditional code; the project already targets both netstandard2.0 and net8.0.

Test Status

Build and full test suite pass:

• ✅ Format check (Fantomas)
• ✅ Build (netstandard2.0 + net8.0)
• ✅ All tests pass (2909+ core, 489 design-time, 292 integration, 2 reference)

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