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/paging.md

@@ -7,7 +7,7 @@ which live in memory we'd like to reclaim and don't control), switches CR3 onto
 them, and — crucially — maps with **real permissions**.
 
 It's x86_64-specific (the 4-level table format is an Intel/AMD thing), so it lives
-behind the [arch](arch.md) boundary in `system/kernel/architecture/x86_64/paging.zig`.
+behind the [architecture](architecture.md) boundary in `system/kernel/architecture/x86_64/paging.zig`.
 
 ## The format