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.3

@@ -0,0 +1,37 @@
+.TH buffer_init_staticcontents 3
+.SH NAME
+buffer_init_staticcontents \- initialize buffer structure
+.SH SYNTAX
+.B #include <libowfat/buffer.h>
+
+void \fBbuffer_init_staticcontents\fR(buffer &\fIb\fR,
+                char* \fIy\fR, size_t \fIylen\fR);
+
+void \fBbuffer_init_staticcontents_forread\fR(buffer &\fIb\fR,
+                const char* \fIy\fR, size_t \fIylen\fR);
+
+.SH DESCRIPTION
+buffer_init_staticcontents sets up a buffer pointing to a fixed buffer.
+No file is associated with the buffer. No resizing is done on the
+buffer. Writing past capacity will return an error.
+
+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/byte.h>
+  #include <assert.h>
+
+  char buf[4096];
+  buffer output;
+
+  char x;
+  buffer_init_staticcontents(&output,buf,sizeof buf);
+  buffer_putsflush(&output,"foo\\nbar\\n");
+  assert(output.p==8 && byte_equal(buf,8,"foo\\nbar\\n"));
+
+.SH "SEE ALSO"
+buffer_init(3), buffer(3), buffer_init_write(3)