Reject write operations with expect_gen=0

Per PROTOCOL.txt specification, WRITE operations must provide a valid
generation counter and cannot use expect_gen=0 (unlike other operations).

This ensures the client has retrieved the file's metadata and knows the
correct chunk size before writing.

Changes:
- Added FILETREE_BADGEN error code (-8)
- Modified file_tree_write() to reject expect_gen=0
- Added error message for FILETREE_BADGEN

src/file_tree.h

@@ -13,6 +13,7 @@ enum {
     FILETREE_EXISTS  = -5,
     FILETREE_BADPATH = -6,
     FILETREE_BADOP   = -7,
+    FILETREE_BADGEN  = -8,
 };
 
 typedef struct Entity Entity;