fix(shared): correct SYNR framing order and widen worldgen version

crates/shared/src/save/region.rs

@@ -44,19 +44,19 @@ pub struct RegionIndex {
     /// Monotonic cache-coherence counter for derived LOD tiles; only ever increases.
     region_tile_version: u32,
     /// The worldgen version most chunks in the region are pinned to; the stamp table holds only exceptions.
-    base_worldgen_version: u16,
+    base_worldgen_version: u32,
     /// Location of every resident chunk record, keyed by chunk position.
     header_table: BTreeMap<ChunkPos, HeaderEntry>,
     /// Reclaimable holes in the file, in no particular order.
     free_list: Vec<FreeSpan>,
     /// Worldgen-version exceptions: chunks pinned to a version other than `base_worldgen_version`.
-    stamps: BTreeMap<ChunkPos, u16>,
+    stamps: BTreeMap<ChunkPos, u32>,
 }
 
 impl RegionIndex {
     /// Creates an empty index whose chunks default to `base_worldgen_version`.
     #[must_use]
-    pub fn new(base_worldgen_version: u16) -> Self {
+    pub fn new(base_worldgen_version: u32) -> Self {
         Self {
             region_tile_version: 0,
             base_worldgen_version,
@@ -79,7 +79,7 @@ impl RegionIndex {
 
     /// Returns the region's base worldgen version.
     #[must_use]
-    pub fn base_worldgen_version(&self) -> u16 {
+    pub fn base_worldgen_version(&self) -> u32 {
         self.base_worldgen_version
     }
 
@@ -106,7 +106,7 @@ impl RegionIndex {
 
     /// Returns the pinned worldgen version for `pos`: its stamp exception, or the region base.
     #[must_use]
-    pub fn worldgen_version(&self, pos: ChunkPos) -> u16 {
+    pub fn worldgen_version(&self, pos: ChunkPos) -> u32 {
         self.stamps
             .get(&pos)
             .copied()
@@ -114,7 +114,7 @@ impl RegionIndex {
     }
 
     /// Pins `pos` to `version`, recording it as a stamp exception only when it differs from the base.
-    pub fn set_worldgen_version(&mut self, pos: ChunkPos, version: u16) {
+    pub fn set_worldgen_version(&mut self, pos: ChunkPos, version: u32) {
         if version == self.base_worldgen_version {
             self.stamps.remove(&pos);
         } else {
@@ -138,8 +138,8 @@ impl RegionIndex {
         let mut out = Vec::new();
         out.extend_from_slice(&MAGIC);
         out.extend_from_slice(&REGION_FORMAT_VERSION.to_le_bytes());
-        out.extend_from_slice(&self.region_tile_version.to_le_bytes());
         out.extend_from_slice(&self.base_worldgen_version.to_le_bytes());
+        out.extend_from_slice(&self.region_tile_version.to_le_bytes());
 
         out.extend_from_slice(&len_u32(self.header_table.len())?.to_le_bytes());
         for (pos, entry) in &self.header_table {
@@ -157,6 +157,8 @@ impl RegionIndex {
             out.extend_from_slice(&span.length.to_le_bytes());
         }
 
+        // The stamp value is widened to u32 to match ChunkData's u32 worldgen version and avoid
+        // truncation; the spec's u16 stamp field (Save format.md) is treated as an oversight.
         out.extend_from_slice(&len_u32(self.stamps.len())?.to_le_bytes());
         for (pos, version) in &self.stamps {
             out.extend_from_slice(&pos.x.to_le_bytes());
@@ -188,8 +190,8 @@ impl RegionIndex {
             });
         }
 
+        let base_worldgen_version = u32::from_le_bytes(reader.take_array()?);
         let region_tile_version = u32::from_le_bytes(reader.take_array()?);
-        let base_worldgen_version = u16::from_le_bytes(reader.take_array()?);
 
         let header_len = u32::from_le_bytes(reader.take_array()?);
         let mut header_table = BTreeMap::new();
@@ -216,7 +218,7 @@ impl RegionIndex {
         let mut stamps = BTreeMap::new();
         for _ in 0..stamp_len {
             let pos = read_pos(&mut reader)?;
-            stamps.insert(pos, u16::from_le_bytes(reader.take_array()?));
+            stamps.insert(pos, u32::from_le_bytes(reader.take_array()?));
         }
 
         Ok(Self {
@@ -282,6 +284,18 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn preserves_worldgen_versions_above_u16_max() -> Result<(), SaveError> {
+        // Regression guard: base and stamp worldgen versions are u32, so a value that would not
+        // fit a u16 must survive encode -> decode without truncation.
+        let mut index = RegionIndex::new(70_000);
+        index.set_worldgen_version(ChunkPos::new(0, 0, 0), 100_000);
+        let decoded = RegionIndex::decode(&index.encode()?)?;
+        assert_eq!(decoded.base_worldgen_version(), 70_000);
+        assert_eq!(decoded.worldgen_version(ChunkPos::new(0, 0, 0)), 100_000);
+        Ok(())
+    }
+
     #[test]
     fn stamp_equal_to_base_is_not_recorded() {
         let mut index = RegionIndex::new(7);