docs: complete the release delta in the changelog and readme

docs/ARCHITECTURE.md

@@ -119,6 +119,20 @@ identifier is a register or a label is an *architecture* question, so it is
 left to `arch` and resolved in the lint/lsp layers.  This keeps the parser
 arch-agnostic and its output deterministic.
 
+### Optional preprocessing
+
+With `Options{Expand: true}` the parser runs a pre-parse pass
+(`preproc.go`) that splices `#include` files (the source directory, then the
+`-I` directories), expands object and parameterised `#define` macros,
+applies `#undef` and the `#ifdef`/`#ifndef`/`#else`/`#endif` family, and
+folds constant expressions left in operands.  The go command's platform
+macros (`GOARCH_<arch>`, `GOOS_<goos>`) arrive through `Options.Predefines`.
+The assembly path (`asm`, `diff`, `audit`) expands; `lint`, `fmt` and the
+language server read the raw file.  The command layer adds the go_asm.h
+generator (`asmhdr.go`): a file that includes go_asm.h gets the package's
+defines type-checked out of its Go files for the target architecture and
+GOOS, with no compiler in the loop.
+
 ### `arch`
 
 Register files are generated programmatically (the regular `R8`-`R15`,