naming: shm → shared_memory — 'shm' is a Unix clipping, not an acronym

Per docs/coding-standards.md (no Unix-abbreviation exception): syscalls
shared_memory_create/map/physical, kernel SharedMemoryObject + handlers,
runtime.shared_memory (library/runtime/shared-memory.zig), the
shared-memory-server/-client test services, the shared-memory QEMU case,
and docs incl. vdso.md's danos_shared_memory_*. 87/87 QEMU tests pass.

system/services/display/display.zig

@@ -307,10 +307,10 @@ fn verifyNativePresent() void {
 fn attachScanout(stride: u32, width: u32, height: u32, format: u32, refresh_hz: u32, capability: ?ipc.Handle, reply: []u8) usize {
     const cap = capability orelse return fail(reply);
     if (width == 0 or height == 0 or stride < width) return fail(reply);
-    const mapped = runtime.shm.map(cap) orelse return fail(reply);
+    const mapped = runtime.shared_memory.map(cap) orelse return fail(reply);
     const scanout = ipc.lookup(.scanout) orelse return fail(reply);
     // A second announce means the driver died and was restarted (V6): re-attach to its fresh
-    // scanout. (The previous shared mapping leaks — there is no shm_unmap syscall yet — but the
+    // scanout. (The previous shared mapping leaks — there is no shared_memory_unmap syscall yet — but the
     // frames are the dead driver's, reclaimed on its exit; a handful across a crash is benign.)
     const reattach = switch (backend) {
         .virtio => true,