disallow arbitrary data gap between header and first block

src/file.rs

@@ -25,7 +25,9 @@ u128 resource_uuid
 
 //global block layout (variable size)
 u64 num_blocks
-for block_id in 0..num_blocks{
+//the start of the first block is implicitly after the global header (32)
+//num_blocks+1 used in Header.block_location is implicitly the end of the file
+for block_id in 1..num_blocks{
 	u64 first_byte
 }