redox-driver-core: add register_driver_shared for Arc-shared drivers (N18 Q1)

Adds an additive `register_driver_shared(Arc<dyn Driver>)` method
to `DeviceManager` so the caller (driver-manager) can pass a
pre-shared `Arc<DriverConfig>` and retain a `Weak` reference
to the SAME allocation. The SIGCHLD reaper's `Weak::upgrade()`
then resolves to the same maps that `probe()` populates, fixing
the long-standing Q1 dead-driver-leak where the reaper operated
on stale clones with empty maps.

local/recipes/drivers/redox-driver-core/source/src/manager.rs

@@ -115,6 +115,16 @@ impl DeviceManager {
             .sort_by(|left, right| right.priority().cmp(&left.priority()));
     }
 
+    /// Register a driver via a pre-shared `Arc<dyn Driver>`. The caller
+    /// retains a clone/weak to the SAME allocation, so internal state
+    /// mutated through `&self` (e.g. DriverConfig's spawned/pid_to_device
+    /// maps) is observable by the caller. Used by driver-manager so the
+    /// SIGCHLD reaper sees the maps that `probe` actually populates.
+    pub fn register_driver_shared(&mut self, driver: std::sync::Arc<dyn Driver>) {
+        self.drivers.push(driver);
+        self.drivers.sort_by(|l, r| r.priority().cmp(&l.priority()));
+    }
+
     /// Add a dynamic-ID entry for `driver_name`. The driver must already be
     /// registered (or be a candidate for deferred registration). The entry
     /// persists until [`remove_dynid`](Self::remove_dynid) or until the