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/vision.md

@@ -99,7 +99,7 @@ own page tables — plus a [test harness](testing.md).
   resource cleanup on death, then a restartable driver as proof. Needs isolation.
   See [resilience.md](resilience.md).
 - **ARM track** — the `aarch64` port so danos runs on the Zero 2 W and Pi 5. Largely
-  independent of the others (it's the [arch layer](arch.md)); directly serves the win
+  independent of the others (it's the [architecture layer](architecture.md)); directly serves the win
   condition. Likely via aarch64-UEFI first (QEMU `virt` + AAVMF), then real boards.
   See [arm.md](arm.md), and [discovery.md](discovery.md) for the device tree it needs.
 - **GUI track** — a framebuffer-based windowing/compositor, and the input + display