fix(P3): defer paging activation to P4 (needs trap handler for debugging)

The page table structure is fully built and identity-mapped, but
activate_page_table() causes a silent hang because no stvec trap
handler is set up yet to report page faults. Deferred to P4 when
trap handling is implemented.

Also fix unnecessary unsafe block warning for riscv interrupt::disable().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Niu Zhihong <zhihong@nzhnb.com>

Author: MRNIU

src/memory/mod.rs

@@ -85,9 +85,11 @@ pub fn memory_init() {
         mem_start + mem_size
     );
 
-    // Step 4: Enable paging
-    unsafe { page_table::activate_page_table(&pt) };
-    log::info!("MemoryInit: paging enabled");
+    // Step 4: Paging activation deferred to P4 (requires trap handler to debug page faults).
+    // The page table structure is fully built and ready.
+    // TODO(P4): after setting up stvec trap handler, call:
+    //   unsafe { page_table::activate_page_table(&pt) };
+    log::info!("MemoryInit: page table ready (activation deferred to P4)");
 
     // Leak the kernel page table — it must live forever
     core::mem::forget(pt);

src/sync/spinlock.rs

@@ -31,7 +31,7 @@ mod interrupt_ops {
 
     #[inline(always)]
     pub fn enable() {
-        riscv::interrupt::supervisor::enable();
+        unsafe { riscv::interrupt::supervisor::enable() };
     }
 }