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/scheduling.md

@@ -7,7 +7,7 @@ chosen for [real-time](vision.md) — it's predictable (you can reason about whi
 task runs when) and its decisions are O(1), unlike a fair-share scheduler.
 
 The scheduler proper (`system/kernel/sched.zig`) is generic; the context switch and new-task
-stack setup are architecture-specific (`system/kernel/architecture/x86_64/`, see [arch](arch.md)).
+stack setup are architecture-specific (`system/kernel/architecture/x86_64/`, see [architecture](architecture.md)).
 
 ## Tasks