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/frame-allocator.md

@@ -10,7 +10,7 @@ per-process memory all ultimately ask the frame allocator for pages.
 
 It's **generic kernel code**: it operates on the neutral `system.MemoryRegion`
 array, so there's no UEFI in it and nothing architecture-specific beyond the 4 KiB
-page. (Contrast [arch.md](arch.md), which is where CPU-specific code lives.)
+page. (Contrast [architecture.md](architecture.md), which is where CPU-specific code lives.)
 
 ## Why a bitmap