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/device-interrupts.md

@@ -10,7 +10,7 @@ back — the same mechanism a scheduler will later use to preempt tasks.
 
 The first device we bring up is the **timer**, because it's the simplest: it lives
 entirely on the CPU's local interrupt controller, needing no external routing.
-It's all x86_64-specific, behind the [arch](arch.md) boundary.
+It's all x86_64-specific, behind the [architecture](architecture.md) boundary.
 
 ## The APIC, not the PIC