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.

docs/README.md

@@ -107,7 +107,7 @@ Start with the north star:
 - **[threading.md](threading.md) — threads, the std-shaped way.** **Built** (M1–M6):
   `runtime.Thread` mirrors `std.Thread`'s API (spawn/join/detach, Mutex/Condition/
   Semaphore) over a **private** thread ABI — several tasks sharing one address space via
-  a `thread_spawn` syscall, futex-backed blocking, aspace refcounting. Why it's the
+  a `thread_spawn` syscall, futex-backed blocking, address-space refcounting. Why it's the
   native type and not literal `std.Thread` (the [private ABI](syscall.md)), and why
   threads stay a narrow opt-in against the [resilience](resilience.md) default. Build
   plan + gates: [threading-plan.md](threading-plan.md).