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/vdso.md

@@ -72,7 +72,7 @@ into every process's address space. The blob is:
 - **Architecture-specific.** The x86-64 blob wraps `syscall`; an aarch64 blob
   wraps `svc #0`. It lives beside the other per-architecture kernel sources
   (`system/kernel/architecture/<arch>/`), selected the same way the
-  `architecture` module is (docs/arch.md).
+  `architecture` module is (docs/architecture.md).
 
 ### Shape: a function table, not an ELF