Switch savegame serialization format from Bincode to custom parsing

- Replaced Bincode-based serialization/deserialization with a custom save format for better control.
- Introduced save header with magic bytes, timestamp handling, and UTF-8 validation.
- Enhanced error handling for save parsing and increased format flexibility.
- Removed

Software/MainBoard/rust/src/hal/savegame_manager.rs

@@ -1,5 +1,4 @@
 use alloc::vec::Vec;
-use bincode::{Decode, Encode};
 use embedded_savegame::storage::{Flash, Storage};
 use embedded_storage::nor_flash::{NorFlash, ReadNorFlash};
 use esp_bootloader_esp_idf::partitions::{Error as PartitionError, FlashRegion};
@@ -25,13 +24,44 @@ pub struct SaveInfo {
     pub created_at: Option<alloc::string::String>,
 }
 
-/// Wrapper that includes both the config data and metadata like creation timestamp.
-#[derive(Serialize, Debug, Encode, Decode)]
-struct SaveWrapper {
-    /// UTC timestamp in RFC3339 format
-    created_at: alloc::string::String,
-    /// Raw config JSON data
-    data: Vec<u8>,
+const SAVE_MAGIC: [u8; 4] = *b"SGM1";
+const SAVE_HEADER_LEN: usize = 6; // magic (4) + timestamp_len (u16)
+
+struct ParsedSave<'a> {
+    created_at: &'a str,
+    data: &'a [u8],
+}
+
+fn parse_save(data: &[u8]) -> FatResult<ParsedSave<'_>> {
+    if data.len() < SAVE_HEADER_LEN {
+        return Err(FatError::String {
+            error: "Save payload too short".into(),
+        });
+    }
+    if data[..4] != SAVE_MAGIC {
+        return Err(FatError::String {
+            error: "Save payload has invalid magic".into(),
+        });
+    }
+
+    let timestamp_len = u16::from_le_bytes([data[4], data[5]]) as usize;
+    let timestamp_end = SAVE_HEADER_LEN + timestamp_len;
+    if timestamp_end > data.len() {
+        return Err(FatError::String {
+            error: "Save payload timestamp length exceeds data".into(),
+        });
+    }
+
+    let created_at = core::str::from_utf8(&data[SAVE_HEADER_LEN..timestamp_end]).map_err(|e| {
+        FatError::String {
+            error: alloc::format!("Save payload contains invalid timestamp UTF-8: {e:?}"),
+        }
+    })?;
+
+    Ok(ParsedSave {
+        created_at,
+        data: &data[timestamp_end..],
+    })
 }
 
 // ── Flash adapter ──────────────────────────────────────────────────────────────
@@ -134,11 +164,21 @@ impl SavegameManager {
     /// slot before `append()` writes to the next free one.
     /// Both operations are performed atomically on the same Storage instance.
     pub fn save(&mut self, data: &[u8], timestamp: &str) -> FatResult<()> {
-        let wrapper = SaveWrapper {
-            created_at: alloc::string::String::from(timestamp),
-            data: data.to_vec(),
-        };
-        let mut serialized = bincode::encode_to_vec(&wrapper, bincode::config::standard())?;
+        let timestamp_bytes = timestamp.as_bytes();
+        let timestamp_len: u16 =
+            timestamp_bytes
+                .len()
+                .try_into()
+                .map_err(|_| FatError::String {
+                    error: "Timestamp too long for save header".into(),
+                })?;
+
+        let mut serialized =
+            Vec::with_capacity(SAVE_HEADER_LEN + timestamp_bytes.len() + data.len());
+        serialized.extend_from_slice(&SAVE_MAGIC);
+        serialized.extend_from_slice(&timestamp_len.to_le_bytes());
+        serialized.extend_from_slice(timestamp_bytes);
+        serialized.extend_from_slice(data);
 
         // Flash storage often requires length to be a multiple of 4.
         let padding = (4 - (serialized.len() % 4)) % 4;
@@ -169,12 +209,8 @@ impl SavegameManager {
         match self.storage.read(idx, &mut buf)? {
             None => Ok(None),
             Some(data) => {
-                // Try to deserialize as SaveWrapper (new Bincode format)
-                let (wrapper, _) = bincode::decode_from_slice::<SaveWrapper, _>(
-                    data,
-                    bincode::config::standard(),
-                )?;
-                Ok(Some(wrapper.data))
+                let parsed = parse_save(data)?;
+                Ok(Some(parsed.data.to_vec()))
             }
         }
     }
@@ -192,17 +228,22 @@ impl SavegameManager {
         let mut buf = alloc::vec![0u8; SAVEGAME_SLOT_SIZE];
         for idx in 0..SAVEGAME_SLOT_COUNT {
             if let Some(data) = self.storage.read(idx, &mut buf)? {
-                // Try to deserialize as SaveWrapper (new Bincode format)
-                let (wrapper, _) = bincode::decode_from_slice::<SaveWrapper, _>(
-                    data,
-                    bincode::config::standard(),
-                )?;
-
-                saves.push(SaveInfo {
-                    idx,
-                    len: wrapper.data.len() as u32,
-                    created_at: Some(wrapper.created_at),
-                });
+                match parse_save(data) {
+                    Ok(save) => {
+                        saves.push(SaveInfo {
+                            idx,
+                            len: save.data.len() as u32,
+                            created_at: Some(alloc::string::String::from(save.created_at)),
+                        });
+                    }
+                    Err(err) => {
+                        saves.push(SaveInfo {
+                            idx,
+                            len: 0,
+                            created_at: None,
+                        });
+                    }
+                }
             }
         }
         Ok(saves)