Add Base.show for StanBlocksError to fix VS Code error display

VS Code's Julia extension uses show() not showerror() for error rendering,
causing raw struct dumps with Ptr{Nothing} pointers. Delegate to showerror.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nsiccha

src/StanBlocks.jl

@@ -126,6 +126,8 @@ function _expr_trace(frames)
     trace
 end
 
+Base.show(io::IO, e::StanBlocksError) = showerror(io, e)
+
 function _showerror_header(io::IO, e::StanBlocksError)
     print(io, "StanBlocksError [$(e.phase)]: $(e.context)\n")
     print(io, "  Caused by: ")