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/kernel/architecture/x86_64/cpu.zig

@@ -189,8 +189,8 @@ pub fn mapUserDmaInto(root: u64, virtual: u64, physical: u64, len: u64) void {
 }
 
 /// Map shared cacheable RAM into address space `root`: write-back cacheable, RW+NX, and
-/// marked so teardown won't free the frames (they're owned by a refcounted shm object,
-/// freed when its last capability drops). For shm_create/shm_map.
+/// marked so teardown won't free the frames (they're owned by a refcounted shared-memory object,
+/// freed when its last capability drops). For shared_memory_create/shared_memory_map.
 pub fn mapUserSharedInto(root: u64, virtual: u64, physical: u64, len: u64) void {
     paging.mapUserSharedInto(root, virtual, physical, len);
 }