docs: rename arch.md to architecture.md and update all references

Rename the file to match current naming conventions and update all
cross-references throughout the documentation. Update the content to
reflect the current state of the codebase:

- Module is now called "architecture", not "arch"
- Directory is system/kernel/architecture/, not system/kernel/arch/
- Generic kernel file is kernel.zig, not main.zig
- Updated file listing for x86_64 architecture module (added smp.zig,
  per-cpu.zig, ioapic.zig, trampoline.s)
- Updated function list in cpu.zig to include enterUser/userExit and
  address-space management
- Clarified that _start lives in architecture-specific isr.s
- Expanded architecture-specific vs generic comparison table with
  actual current modules

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/memory-map.md

@@ -16,7 +16,7 @@ forward it to the kernel as-is. We don't, for two reasons:
    memory-type numbers and walk the array using UEFI's variable descriptor stride.
    That's UEFI vocabulary bleeding across the handoff — and danos wants to boot on
    systems that have no UEFI at all (a Raspberry Pi describes its memory with a
-   *device tree* instead). See [arch.md](arch.md) for the same "keep the kernel
+   *device tree* instead). See [architecture.md](architecture.md) for the same "keep the kernel
    platform-agnostic" principle applied to CPU code.
 2. **We already established the better pattern.** The loader doesn't hand the
    kernel a raw UEFI GOP either — [`queryFramebuffer`](gop.md) converts it to