The fused per-file compile pipeline.
One arena, one parse, one fused transform pass-set, one codegen: TSX/TS/JSX -> strip types -> JSX (automatic runtime) -> React Fast Refresh instrumentation (dev) -> JS + sourcemap.
This is the hot path of the whole tool. Everything here must stay allocation-conscious and must never re-parse between stages — that single-parse property is the speed edge over plugin-pipeline designs.