gcc 15 and C23 force some union trickery on buffer.h :-(

add a few buffer_init*_forread variants to pretend we have type safety
  make sure buffer_init_staticcontents handles flushing attempts

buffer/buffer_init_staticcontents_forread.3

@@ -0,0 +1,32 @@
+.TH buffer_init_staticcontents_forread 3
+.SH NAME
+buffer_init_staticcontents_forread \- initialize buffer structure
+.SH SYNTAX
+.B #include <libowfat/buffer.h>
+
+void \fBbuffer_init_staticcontents_forread\fR(buffer &\fIb\fR,
+                const char* \fIy\fR, size_t \fIylen\fR);
+
+.SH DESCRIPTION
+buffer_init_staticcontents_forread sets up a buffer for reading,
+pointing to a pre-filled fixed buffer.
+
+This is useful for writing unit tests.
+
+.SH EXAMPLE
+  #include <libowfat/buffer.h>
+  #include <libowfat/open.h>
+
+  char buf[]="foo\\nbar\\n";
+  buffer input;
+
+  char x;
+  buffer_init_staticcontents_forread(&input,buf,sizeof buf);
+  while (buffer_get(&input,&x,1)==1) {
+    buffer_put(buffer_1,&x,1);
+    if (x=='\\n') break;
+  }
+  buffer_flush(buffer_1);
+
+.SH "SEE ALSO"
+buffer_init(3), buffer(3), buffer_init_staticcontents(3)