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()
+            }
         }
     }