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/vdso.md

@@ -119,7 +119,7 @@ One table entry per kernel call, C ABI (System V AMD64), names prefixed
 returns are `u64`, errors return as negative values exactly as today.
 
 The calls that return two values in `rax:rdx` today — `dma_alloc`
-(vaddr + paddr), `msi_bind` (address + data), `shm_create` (vaddr + handle) —
+(virtual_address + physical_address), `msi_bind` (address + data), `shm_create` (virtual_address + handle) —
 become functions returning a two-`u64` struct. The System V ABI returns a
 16-byte struct in `rax:rdx`, so the stub is a plain `syscall; ret` — the
 C-ABI spelling of the existing convention, at zero cost.