Stabilize DRM core contracts: fix latent panics, add diagnostics and tests

Production code fixes:
- scheme.rs: replace unwrap() after checked_mul with match binding,
  eliminating a latent panic if code is reordered
- main.rs: log request context_id (PID) on request handling failure
  instead of silently discarding the error
- drivers/amd/display.rs: split silent EDID read fallback into
  separate match arms with log::warn diagnostics for short reads
  and read failures, including byte count and connector index

Test coverage:
- gem.rs: add 4 basic tests for GemManager (create+verify,
  close+verify removal, double-close error, invalid handle error)

local/recipes/gpu/redox-drm/source/src/drivers/amd/display.rs

@@ -338,7 +338,22 @@ impl DisplayCore {
 
         match self.read_edid_block(connector_index, 0x00) {
             Ok(edid) if edid.len() >= 128 => edid,
-            Ok(_) | Err(_) => Vec::new(),
+            Ok(short) => {
+                log::warn!(
+                    "redox-drm: short EDID ({} bytes) from AMD connector {}",
+                    short.len(),
+                    connector_index
+                );
+                Vec::new()
+            }
+            Err(e) => {
+                log::warn!(
+                    "redox-drm: EDID read failed for AMD connector {}: {}",
+                    connector_index,
+                    e
+                );
+                Vec::new()
+            }
         }
     }
 

local/recipes/gpu/redox-drm/source/src/gem.rs

@@ -121,3 +121,42 @@ impl GemManager {
         Ok(self.object(handle)?.gpu_addr)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn create_and_object_exists() {
+        let mut mgr = GemManager::new();
+        let h = mgr.create(4096).expect("create should succeed");
+        let obj = mgr.object(h).expect("object should exist after create");
+        assert_eq!(obj.handle, h);
+        assert_eq!(obj.size, 4096);
+    }
+
+    #[test]
+    fn close_removes_object() {
+        let mut mgr = GemManager::new();
+        let h = mgr.create(4096).expect("create should succeed");
+        mgr.close(h).expect("close should succeed");
+        assert!(mgr.object(h).is_err(), "object should be gone after close");
+    }
+
+    #[test]
+    fn double_close_returns_error() {
+        let mut mgr = GemManager::new();
+        let h = mgr.create(4096).expect("create should succeed");
+        mgr.close(h).expect("first close should succeed");
+        assert!(mgr.close(h).is_err(), "second close should fail");
+    }
+
+    #[test]
+    fn object_by_invalid_handle_returns_error() {
+        let mgr = GemManager::new();
+        assert!(
+            mgr.object(99999).is_err(),
+            "querying a non-existent handle should fail"
+        );
+    }
+}

local/recipes/gpu/redox-drm/source/src/main.rs

@@ -129,8 +129,11 @@ fn run() -> Result<()> {
 
         let response = match response {
             Ok(response) => response,
-            Err(_request) => {
-                error!("redox-drm: failed to handle request");
+            Err(request) => {
+                error!(
+                    "redox-drm: failed to handle request from context {}",
+                    request.context_id()
+                );
                 continue;
             }
         };

local/recipes/gpu/redox-drm/source/src/scheme.rs

@@ -1011,14 +1011,16 @@ impl DrmScheme {
                     );
                     return Err(Error::new(EINVAL));
                 }
-                let required_size = (pitch as u64).checked_mul(req.height as u64);
-                if required_size.is_none() {
-                    warn!(
-                        "redox-drm: ADDFB pitch * height overflows pitch={} height={}",
-                        pitch, req.height
-                    );
-                    return Err(Error::new(EINVAL));
-                }
+                let required_size = match (pitch as u64).checked_mul(req.height as u64) {
+                    Some(s) => s,
+                    None => {
+                        warn!(
+                            "redox-drm: ADDFB pitch * height overflows pitch={} height={}",
+                            pitch, req.height
+                        );
+                        return Err(Error::new(EINVAL));
+                    }
+                };
                 let owned = self
                     .handles
                     .get(&id)
@@ -1035,10 +1037,10 @@ impl DrmScheme {
                     warn!("redox-drm: ADDFB handle {} not found: {}", req.handle, e);
                     Error::new(ENOENT)
                 })?;
-                if required_size.unwrap() > actual_size {
+                if required_size > actual_size {
                     warn!(
                         "redox-drm: ADDFB requires {} bytes but GEM {} is {} bytes",
-                        required_size.unwrap(),
+                        required_size,
                         req.handle,
                         actual_size
                     );