kernel: implement /proc/[pid]/statm — memory summary

New ProcStatm ContextHandle that reports the 7-field memory summary
matching Linux /proc/[pid]/statm format:

  size resident shared text lib data dt

All values in pages. Cross-referenced with Linux 7.1
fs/proc/array.c:proc_pid_statm().

Resident count is approximated as the total physical grants
(GRANT_PHYS), since Red Bear has no paging/swapping and all
memory is resident. Other fields (shared, lib, dt) are 0 since
Red Bear's memory model doesn't distinguish these.

src/scheme/proc.rs

@@ -173,6 +173,9 @@ enum ContextHandle {
     ProcMaps {
         pid: usize,
     },
+    ProcStatm {
+        pid: usize,
+    },
 }
 #[derive(Clone)]
 struct Handle {
@@ -393,6 +396,7 @@ impl ProcScheme {
             (Some("cmdline"), None) => Some(ContextHandle::ProcCmdline { pid }),
             (Some("status"), None) => Some(ContextHandle::ProcStatus { pid }),
             (Some("maps"), None) => Some(ContextHandle::ProcMaps { pid }),
+            (Some("statm"), None) => Some(ContextHandle::ProcStatm { pid }),
             _ => None,
         }
     }
@@ -1930,6 +1934,34 @@ impl ContextHandle {
                 }
                 read_from(buf, output.as_bytes(), offset)
             }
+            ContextHandle::ProcStatm { pid } => {
+                let context = proc_context(*pid, token).ok_or(Error::new(ESRCH))?;
+                let addr_space = context.read(token.token()).addr_space.clone();
+                let (size, resident) = if let Some(addr_space) = addr_space {
+                    let token2 = token.downgrade();
+                    let guard = addr_space.acquire_read(token2);
+                    let mut total_size = 0usize;
+                    let mut total_resident = 0usize;
+                    for (_grant_base, grant_info) in guard.grants.iter() {
+                        let page_count = grant_info.page_count() * PAGE_SIZE;
+                        total_size += page_count;
+                        if grant_info.grant_flags().contains(GrantFlags::GRANT_PHYS) {
+                            total_resident += page_count;
+                        }
+                    }
+                    (total_size / PAGE_SIZE, total_resident / PAGE_SIZE)
+                } else {
+                    (0, 0)
+                };
+                // Format: size resident shared text lib data dt (in pages)
+                // Cross-referenced with Linux 7.1 fs/proc/array.c:proc_pid_statm().
+                // On Red Bear we approximate: shared=0, text=size, lib=0, data=size, dt=0.
+                let output = alloc::format!(
+                    "{} {} 0 {} 0 {} 0\n",
+                    size, resident, size, size
+                );
+                read_from(buf, output.as_bytes(), offset)
+            }
             ContextHandle::ProcRoot | ContextHandle::ProcDir { .. } => Err(Error::new(EBADF)),
             ContextHandle::MmapMinAddr(addrspace) => {
                 let mut token = token.token();