feat(parser): macro expansion, conditionals and include splicing with -I

Assisted-by: GLM 5.3 Flash

docs/man/gasm-diff.1

@@ -2,7 +2,7 @@
 .SH NAME
 gasm-diff \- compare the machine code of two assembly files
 .SH SYNOPSIS
-.B gasm diff [\-GOARCH arch] <file1.s> <file2.s>
+.B gasm diff [\-GOARCH arch] [\-I dir] <file1.s> <file2.s>
 .SH DESCRIPTION
 Compare the machine code produced by assembling two files.  Shows which
 functions differ and the byte-level differences.  Useful for verifying
@@ -20,6 +20,10 @@ pairs two variants regardless of suffix.
 Target architecture for both files: amd64, arm64, riscv64 or loong64;
 overrides the file-name suffixes.
 .TP
+.B \-I \fIdir\fR
+Directory to search for #include files; may be repeated, searched in
+order after the source directory.
+.TP
 .B \-\-map \fIspec\fR
 Comma-separated old=new pairs to match functions with different names.
 .SH EXIT STATUS