kernel: replace OOM panic with graceful log-and-continue

Previously: PfError::Oom during page fault recovery caused the
entire kernel to panic via todo!("oom"). This means a single
OOM allocation in a user process would crash the whole OS.

Now: OOM is logged as a warning, the faulting process receives
SIGSEGV (same path as other non-fatal page faults), and the
kernel continues. The process may be killed, but the system
stays up. This is consistent with how Linux handles OOM during
page fault handling: send SIGSEGV/SIGKILL, don't panic.

src/memory/mod.rs

@@ -1027,7 +1027,13 @@ pub fn page_fault_handler(
         let mut token = unsafe { CleanLockToken::new() };
         match context::memory::try_correcting_page_tables(faulting_page, mode, &mut token) {
             Ok(()) => return Ok(()),
-            Err(PfError::Oom) => todo!("oom"),
+            Err(PfError::Oom) => {
+                log::warn!("page fault recovery failed: out of memory at {:#x}", faulting_page.start_address().data());
+                // OOM during page table correction is non-fatal: the process
+                // receives SIGSEGV but the kernel continues. The faulting
+                // allocation will be retried or the process killed by the OOM
+                // killer when it's implemented.
+            }
             Err(PfError::Segv | PfError::RecursionLimitExceeded | PfError::NonfatalInternalError) => (),
         }
     }