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.

test/qemu_test.py

@@ -295,8 +295,8 @@ CASES = [
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
 
     # docs/threading-plan.md M1: address-space refcount — spaces destroyed exactly
-    # once per process, no leak/double-free (the foundation shared-aspace threads need).
-    {"name": "aspace-refcount",
+    # once per process, no leak/double-free (the foundation shared-address-space threads need).
+    {"name": "address-space-refcount",
      "timeout": 60,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},