Apply C/C++ struct reorderings in-place, returning the modified source.
Each layout in layouts is looked up by name; matched structs are replaced
with the optimally-ordered definition. Replacements are applied back-to-front
so byte offsets remain valid.
Render a reordered Zig struct definition as source text.
Zig structs are declared as const Name = struct { ... };.
If the layout is packed, the output uses packed struct.