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/sysv.md

@@ -116,7 +116,7 @@ has to speak that same convention at the point of the call.
 
 ## A note on other architectures
 
-This is x86-64-specific. An AArch64 port ([arch.md](arch.md)) has its own calling
+This is x86-64-specific. An AArch64 port ([architecture.md](architecture.md)) has its own calling
 convention (arguments in X0–X7, and so on) — a different ABI entirely. `kernel_abi`
 would be set per-architecture, but the *principle* is the same: the loader/entry
 boundary and the kernel must agree on how arguments are passed.