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/shared-memory-client/shared-memory-client.zig

@@ -0,0 +1,46 @@
+//! system/services/shared-memory-client — the creating half of the shared-memory test (docs/display-v2.md V2).
+//! It `shared_memory_create`s a shared region, writes a known pattern into it, and hands the region's
+//! capability to `shared-memory-server` as an `ipc_call` send_cap. The server maps that capability and
+//! confirms the pattern is visible — proving cross-process shared memory over the extended
+//! capability-passing path.
+
+const runtime = @import("runtime");
+const system = runtime.system;
+const shared_memory = runtime.shared_memory;
+const ipc = runtime.ipc;
+
+const pattern_len = 4096;
+
+/// The pattern the server checks — must match shared-memory-server.zig.
+fn expected(i: usize) u8 {
+    return @truncate(i *% 7 +% 3);
+}
+
+fn lookupServer() ?ipc.Handle {
+    var attempts: usize = 0;
+    while (attempts < 100) : (attempts += 1) {
+        if (ipc.lookup(.shared_memory_test)) |h| return h;
+        system.sleep(50);
+    }
+    return null;
+}
+
+pub fn main() void {
+    const region = shared_memory.create(pattern_len) orelse {
+        _ = system.write("shared-memory: create failed\n");
+        return;
+    };
+    var i: usize = 0;
+    while (i < pattern_len) : (i += 1) region.ptr[i] = expected(i);
+
+    const server = lookupServer() orelse {
+        _ = system.write("shared-memory: no server\n");
+        return;
+    };
+    // A non-empty message (so it reaches on_message, not the ping path), carrying the shared-memory
+    // region's capability. The reply is empty; we just need the round trip.
+    var reply: [64]u8 = undefined;
+    _ = ipc.callCap(server, "shared-memory", &reply, region.handle) catch {
+        _ = system.write("shared-memory: call failed\n");
+    };
+}