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_tosa.c

@@ -1,7 +1,7 @@
 #include "stralloc.h"
 #include "buffer.h"
 
-static ssize_t strallocwrite(int fd,char* buf,size_t len,void* myself) {
+static ssize_t strallocwrite(int fd,const char* buf,size_t len,void* myself) {
   buffer* b=myself;
   stralloc* sa=b->cookie;
   sa->len+=len;
@@ -21,7 +21,7 @@ int buffer_tosa(buffer* b,stralloc* sa) {
   b->n=0;
   b->a=1024;
   b->fd=0;
-  b->op=strallocwrite;
+  b->op.wopc=strallocwrite;
   b->cookie=sa;
   b->deinit=0;
   return 0;