decoder: signed layouts (two's complement, lengths 8 and 16)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

src/decoder/layout.rs

@@ -21,6 +21,8 @@ pub struct Layout {
     pub bit: u32,
     /// Length 16 only: true = `byte` is the most significant.
     pub big: bool,
+    /// Lengths 8 and 16: two's complement (`"signed": true` in the can block).
+    pub signed: bool,
     pub scale: f64,
     pub offset: f64,
     pub kind: Kind,
@@ -48,7 +50,12 @@ impl Layout {
         Some(match self.kind {
             Kind::Bool => Value::Bool(raw != 0),
             Kind::Number => {
-                let v = raw as f64 * self.scale + self.offset;
+                let raw = match (self.signed, self.length) {
+                    (true, 8) => raw as u8 as i8 as f64,
+                    (true, 16) => raw as u16 as i16 as f64,
+                    _ => raw as f64,
+                };
+                let v = raw * self.scale + self.offset;
                 // Three decimals: 0.4 × 98 should read 39.2, not 39.200000000000003.
                 let v = (v * 1000.0).round() / 1000.0;
                 serde_json::Number::from_f64(v).map(Value::Number).unwrap_or(Value::Null)
@@ -97,6 +104,7 @@ pub fn load(registry: &str) -> Result<Vec<Layout>, String> {
             length,
             bit,
             big,
+            signed: c["signed"].as_bool().unwrap_or(false),
             scale: c["scale"].as_f64().unwrap_or(1.0),
             offset: c["offset"].as_f64().unwrap_or(0.0),
             kind,
@@ -136,6 +144,10 @@ mod tests {
         let pedal = &one(json!({"id": "0x210", "byte": 0, "length": 8, "scale": 0.4}), "number").unwrap()[0];
         assert_eq!(pedal.value(&[0xFA]), Some(json!(100.0)));
         assert_eq!(pedal.value(&[98]), Some(json!(39.2)));
+        // Signed: 0C0's steering angle, 0.1° per count, little-endian two's complement.
+        let steer = &one(json!({"id": "0x0C0", "byte": 0, "length": 16, "order": "little", "signed": true, "scale": 0.1}), "number").unwrap()[0];
+        assert_eq!(steer.value(&[0x41, 0xED]), Some(json!(-479.9))); // 0xED41 = -4799
+        assert_eq!(steer.value(&[0x69, 0x12]), Some(json!(471.3))); // 0x1269 = 4713
         let brake = &one(json!({"id": "0x231", "byte": 4, "bit": 1, "length": 1}), "boolean").unwrap()[0];
         assert_eq!(brake.value(&[0, 0, 0, 0, 0x02]), Some(json!(true)));
         assert_eq!(brake.value(&[0, 0, 0, 0, 0xFD]), Some(json!(false)));