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/arm.md

@@ -5,7 +5,7 @@ though — the Pis span **two different CPU architectures** (32-bit `arm` and 64
 `aarch64`) and (stock) a different boot protocol from x86-64's UEFI. **danos targets
 `aarch64` only** (see the decision below); the `arm`/`aarch64` distinction still
 matters for understanding why. This page maps the landscape so the
-[arch split](arch.md) and build system can be planned for it.
+[architecture split](architecture.md) and build system can be planned for it.
 
 ## `arm` vs `aarch64` — 32-bit vs 64-bit
 
@@ -39,7 +39,7 @@ page-table formats, and calling conventions. Each needs its own `system/kernel/a
 
 ## Booting: UEFI is not x86-only
 
-The boot protocol is a **separate axis** from the CPU (see [arch.md](arch.md)):
+The boot protocol is a **separate axis** from the CPU (see [architecture.md](architecture.md)):
 
 - **UEFI** exists for ARM too — ARM servers require it (SBSA/SBBR), QEMU boots it
   with **AAVMF** (the AArch64 build of the same EDK2 firmware as x86's OVMF), and
@@ -114,7 +114,7 @@ Two routes, mirroring how we test x86-64 with OVMF:
 
 ## Related
 
-- [arch.md](arch.md) — the arch-module boundary these targets plug into, and the
+- [architecture.md](architecture.md) — the arch-module boundary these targets plug into, and the
   CPU-arch vs boot-protocol "two axes".
 - [efi.md](efi.md) — the UEFI loader that carries over to aarch64-UEFI.
 - [vision.md](vision.md) — why isolated, portable-across-architectures is the goal.