USB: P0 fix — eliminate runtime panics in usbscsid main loop

IMPROVEMENT-PLAN.md §10.1.6: critical safety fix.

usbscsid main.rs had 3 runtime unwrap sites that would panic
the daemon on transient errors:

1. Line 106: debug block 0 read on init — now uses if-let to
   skip the debug print if the read fails (disconnected device,
   media error). The device still registers its scheme.

2. Line 144: event_queue event unwrap — now handles Err()
   with eprintln + continue instead of panic.

3. Line 147: scheme.tick() unwrap — now handles Err()
   with eprintln instead of panic.

Scheme tick failures propagate gracefully — the event loop
continues, the daemon survives. This matches the Linux 7.1
pattern of logging USB errors without crashing the daemon.

drivers/storage/usbscsid/src/main.rs

@@ -103,8 +103,9 @@ fn daemon(daemon: daemon::Daemon) -> ! {
     }
 
     let mut buffer = [0u8; 512];
-    scsi.read(&mut *protocol, 0, &mut buffer).unwrap();
-    println!("DISK CONTENT: {}", base64::encode(&buffer[..]));
+    if let Ok(()) = scsi.read(&mut *protocol, 0, &mut buffer).map(|_| ()) {
+        println!("DISK CONTENT: {}", base64::encode(&buffer[..]));
+    }
 
     let event_queue = event::EventQueue::new().unwrap();
 
@@ -141,10 +142,19 @@ fn daemon(daemon: daemon::Daemon) -> ! {
         .unwrap();
 
     for event in event_queue {
-        match event.unwrap().user_data {
-            Event::Scheme => driver_block::FuturesExecutor
-                .block_on(scheme.tick())
-                .unwrap(),
+        let event = match event {
+            Ok(e) => e,
+            Err(e) => {
+                eprintln!("usbscsid: event error: {}", e);
+                continue;
+            }
+        };
+        match event.user_data {
+            Event::Scheme => {
+                if let Err(e) = driver_block::FuturesExecutor.block_on(scheme.tick()) {
+                    eprintln!("usbscsid: scheme tick error: {}", e);
+                }
+            }
         }
     }