[dajiaohuang]

Problem

A supported Utf16ValueStringBuilder.Insert can throw ArgumentException: Destination is too short when existing text exceeds the default buffer size. For example, append 33,000 a characters, then call Insert(1, "b"). The new buffer is sized using the insertion index rather than the existing text length, so it cannot hold the retained suffix.

Related: #125 (closed by stale automation).

Change

Size the new buffer from the current length plus the inserted text length. Add BCL-comparison regressions for pooled/thread-static builders and insertion at the start/middle. Update the Unity source through the existing netstandard2.1 post-build copy target.

Validation

• All 4 new cases fail at master b9c1d8a and pass with this change.
• Release solution build passes (existing warnings remain).
• Full maintained test project: 151 passed, using DOTNET_ROLL_FORWARD=Major because .NET 7 runtime is unavailable locally.
• No Unity editor/TMP/IL2CPP execution; no allocation or performance claims.