Summary

• Thread-safe by default: ValueStringBuilder constructors and Create now default to mt: true (BCL ArrayPool<char>.Shared); mt: false is the explicit opt-in to the single-threaded STArrayPool<char>.Shared for hot paths.
• RawInterpolatedStringHandler, PooledArraySpanFormattable, StringHelpers.Remove and ToPooledArray now rent from and return to ArrayPool<char>.Shared (pool pairing traced end to end: every Rent returns to the pool it came from, including the handler buffer recycled through ValueStringBuilder.Append).
• ToPooledArray caller contract updated: return via ArrayPool<char>.Shared.Return(array).
• Docs article "Buffers and pooled strings" decision table and examples aligned; STArrayPool itself is unchanged and still available for direct use.

Test plan

• Full suite green: 1145/1145
• Release build of SquidStd.Core clean (XML doc validation)
• Opt-in path covered by new SingleThreadedMode_UsesSTArrayPoolAndRoundTrips test