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/discovery.md

@@ -5,7 +5,7 @@ MMIO addresses, IRQ numbers, CPU count, and interrupt controller it can't just
 assume. On x86 that description comes from **ACPI** tables; on ARM from a **device
 tree** (DTB). This note is a design plan, not built yet: *when* danos should tackle
 it, and *how* to keep it architecture-agnostic — the same discipline the
-[memory map](memory-map.md) and [arch split](arch.md) already follow.
+[memory map](memory-map.md) and [architecture split](architecture.md) already follow.
 
 ## What the kernel assumes today