docs+code: spell out aspace/vaddr/paddr per coding standards

Expand the abbreviations flagged in docs/coding-standards.md (names spelled
out in full unless an acronym) across the kernel, runtime, ABI, tests, and
docs:

  aspace -> address_space  (AspaceRef -> AddressSpaceRef, retainAspace ->
           retainAddressSpace, loaded_aspace -> loaded_address_space, the
           liveAspaceCount/aspaceDestroyCount test hooks, etc.)
  vaddr  -> virtual_address
  paddr  -> physical_address

The kernel test case and its serial markers are renamed to match:
aspace-refcount -> address-space-refcount (kernel dispatch string and
test/qemu_test.py case name kept in sync). Prose in docs uses the natural
"address space"/"virtual address"; backticked field/identifier references
use the code spelling.

Also expand the bare "AS" abbreviation in three ABI comments and reframe the
set_thread_pointer ABI/handler docs to lead with the arch-neutral concept
(user-space TLS thread pointer; x86_64 IA32_FS_BASE, aarch64 TPIDR_EL0)
rather than x86 FS-first, matching scheduler.zig's existing framing.

Foreign ABI names preserved: the ELF p_vaddr field and mmap/mmio remain.

Verified: zig build, zig build test, and the full 25-case QEMU guardrail
suite all green.

system/services/thread-test/thread-test.zig

@@ -40,7 +40,7 @@ fn runSpawnMode() void {
         runtime.system.yield();
     }
     if (spawn_done.load(.acquire) == 1 and shared_value == sentinel) {
-        write("thread-test: child ran in shared aspace ok\n");
+        write("thread-test: child ran in shared address space ok\n");
     } else {
         write("thread-test: FAIL worker did not update shared memory\n");
     }
@@ -349,7 +349,7 @@ fn runAllocMode() void {
     for (threads[0..alloc_threads]) |t| t.join();
 
     // Every thread must have completed all rounds with each block intact — proof the
-    // shared heap and the per-aspace mmap arena are safe under concurrent allocation.
+    // shared heap and the per-address_space mmap arena are safe under concurrent allocation.
     if (allocs_clean.load(.acquire) != alloc_threads) {
         write("thread-alloc: FAIL corruption or OOM under concurrent allocation\n");
         return;