feat(cmd): generate go_asm.h for package-context assembly

Assisted-by: GLM 5.3 Flash

cmd/gasm/main.go

@@ -503,6 +503,12 @@ system toolchain; goobj emits the Go toolchain's own object format, which
 cmd/link consumes directly (it requires -p, the package path, and the
 installed Go toolchain: the object preamble is captured from go tool asm
 and the format version from go version).
+
+A file that includes go_asm.h gets that header generated automatically from
+the package it lives in (the .go files beside it, type-checked for the
+target architecture, the toolchain's own defines), so GOROOT assembly
+assembles without a compiler.  A package that has no Go files for the
+target or does not type-check is a hard error naming the package.
 `)
 	out := fs.String("o", "", "write the output to this file")
 	format := fs.String("format", "raw", "output format: raw (concatenated image), elf or goobj (Go object)")
@@ -538,6 +544,23 @@ and the format version from go version).
 		fmt.Fprintln(os.Stderr, "gasm:", err)
 		return 1
 	}
+	// A file that includes go_asm.h cannot assemble without the package's
+	// defines, and without a compiler nothing else has generated them, so
+	// gasm produces the equivalent itself: automatic, because the compiler
+	// behaves the same way and a flag would only ever be forgotten.  A
+	// generation failure is fatal and names the package: assembling against
+	// a missing header would fail later with a bare "undefined" instead.
+	// A go_asm.h that already resolves (placed by hand, or passed with -I)
+	// is left alone.
+	if needsGoAsmHeader(src) && !goAsmHeaderResolved(filepath.Dir(path), dirs) {
+		hdrDir, cleanup, err := ensureGoAsmHeader(path, targetArch, nil)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "gasm asm:", err)
+			return 1
+		}
+		defer cleanup()
+		dirs = append(dirs, hdrDir)
+	}
 	f, errs := parser.ParseWithOptions(path, src, parser.Options{Expand: true, IncludeDirs: dirs})
 	for _, e := range errs {
 		fmt.Fprintf(os.Stderr, "%s: %v\n", path, e)