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/README.md

@@ -129,7 +129,7 @@ Cutting across all of these:
   (MBR ESP partition + El Torito EFI entry, one embedded image) that Etcher/dd
   flash to USB or a burner writes to disc — built by an in-repo pure-Python
   tool, like the FAT image itself.
-- **[arch.md](arch.md) — the architecture split.** How CPU-specific code is kept
+- **[architecture.md](architecture.md) — the architecture split.** How CPU-specific code is kept
   behind a build-time `arch` module so the generic kernel never names x86_64,
   leaving room for other systems (e.g. an AArch64 Raspberry Pi) later.
 - **[arm.md](arm.md) — ARM targets.** The Raspberry Pi landscape the arch split is
@@ -180,7 +180,7 @@ brings up the **heap** for dynamic allocation ([heap.md](heap.md)), starts the
 **scheduler** ([scheduling.md](scheduling.md)) and the **timer** that preempts it
 ([device-interrupts.md](device-interrupts.md)) — with tasks blocking, sleeping and
 passing messages over **[IPC](ipc.md)** channels — runs, its CPU-specific bits
-behind the [arch](arch.md) boundary, and when idle, or on a panic, it **halts**
+behind the [architecture](architecture.md) boundary, and when idle, or on a panic, it **halts**
 ([halting.md](halting.md)).
 
 Above that line the microkernel proper begins: **discovery** ([discovery.md](discovery.md),