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.

library/runtime/runtime.zig

@@ -38,9 +38,9 @@ pub const device = @import("device.zig");
 pub const dma = @import("dma.zig");
 
 /// Shared cacheable memory: create a region + capability, pass the capability to another
-/// process (an `ipc_call` send_cap), map the same pages there. See library/runtime/shm.zig
+/// process (an `ipc_call` send_cap), map the same pages there. See library/runtime/shared-memory.zig
 /// and docs/display-v2.md.
-pub const shm = @import("shm.zig");
+pub const shared_memory = @import("shared-memory.zig");
 
 /// USB class-driver client: open a device on the xHCI bus and drive it
 /// (control / interrupt / bulk transfers). See library/runtime/usb.zig.