cube_format: a geometry encodes the version it is, not a constant

V3::encode wrote a hardcoded VERSION_V3. It had no callers, so the mistake
cost nothing — and then userspace's v4 writer became the first caller, and a
v4 geometry would have been published under a v3 header: every reader walking
a 42-byte index 40 bytes at a time, finding a store that is silently wrong.
One wrong byte, found before the first v4 image was written rather than after.

drivers/cube/cube_format.rs

@@ -219,12 +219,18 @@ impl V3 {
 
     /// Write the header this geometry describes. `image_bytes` is filled in by the caller's
     /// arithmetic, since only it knows how long the values are.
+    ///
+    /// The version written is the geometry's own rather than a constant. A v4 geometry has a
+    /// 42-byte index, and a header saying v3 would tell every reader to walk it 40 bytes at a time
+    /// — one wrong byte that silently mis-addresses the whole store. This had no callers when it
+    /// was written, so the mistake cost nothing; it has one now, and finding it before the first
+    /// v4 image is written is the whole value of fixing it here.
     pub fn encode(&self, out: &mut [u8], curve: u8, image_bytes: u64) -> Result<usize, Bad> {
         if out.len() < HEADER_LEN_V3 {
             return Err(Bad::Extent);
         }
         out[0..4].copy_from_slice(MAGIC);
-        out[4] = VERSION_V3;
+        out[4] = self.version;
         out[5] = curve;
         out[6..14].copy_from_slice(&image_bytes.to_le_bytes());
         out[14..22].copy_from_slice(&self.record_count.to_le_bytes());