Fix autoprint losing types in multiline output

indent() for non-StanIO was creating a StanIO, causing StanExpr
show to omit types. Use IOContext instead to preserve the original
IO type while tracking indent level.

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

Author: nsiccha

src/slic_stan/slic.jl

@@ -732,10 +732,10 @@ struct StanIO{P} <: WrappedIO
     StanIO(parent; kwargs...) = new{typeof(parent)}(parent, kwargs)
 end
 remake(io::StanIO, p=parent(io); kwargs...) = StanIO(p; io.info..., kwargs...)
-current_indent(io) = ""
+current_indent(io) = repeat("    ", get(io, :_autoprint_indent, 0))
 current_indent(io::StanIO) = repeat("    ", current_indent_level(io))
 current_indent_level(io::StanIO) = get(io.info, :current_indent_level, 0)
-indent(io) = StanIO(io; current_indent_level=1)
+indent(io) = IOContext(io, :_autoprint_indent => 1 + get(io, :_autoprint_indent, 0))
 indent(io::StanIO) = remake(io; current_indent_level=1+current_indent_level(io))
 maybe_indent(io, x::StanBlock) = indent(io)
 maybe_indent(io, x::FunctionsBlock) = io