Make zig-out a FHS image, and the boot volume

`zig build` now installs into a FHS-shaped zig-out that *is* the danos filesystem
and the boot volume — no more zig-out/bin or a separate esp/:

  zig-out/EFI/BOOT/BOOTX64.efi        (firmware entry; UEFI fixes this path)
  zig-out/boot/initial-ramdisk.img
  zig-out/system/kernel               (the kernel binary)
  zig-out/system/services/init  vfs
  zig-out/system/drivers/hpet   bus

Binaries land at their addressed, leaf-collapsed paths per the sub-project
resolution rule (system/services/init/init.zig -> system/services/init); vfs, hpet,
and bus are installed to their FHS homes too, so the image is complete even though
at boot they arrive inside the initial-ramdisk.

The bootloader (boot/efi.zig) now loads each artifact from its FHS path
(system\kernel, system\services\init, boot\initial-ramdisk.img); run-x86-64 mounts
zig-out directly; the QEMU test harness assembles its ESP from the FHS zig-out.

Also renames system/kernel/main.zig -> kernel.zig so the kernel follows the
name/name.zig convention (kernel/ = ring-0 code, services/ = ring-3 OS services).
Documents the resolution rule in the repository-layout section (README + coding
standard). Suite 35/35 plus host tests green.

system/danos.zig

@@ -1,5 +1,5 @@
 //! Shared definitions that form the contract between a bootloader
-//! (boot/, e.g. efi.zig built as BOOTX64.efi) and the kernel (system/kernel/main.zig).
+//! (boot/, e.g. efi.zig built as BOOTX64.efi) and the kernel (system/kernel/kernel.zig).
 //!
 //! Both binaries import this as the "danos" module, so the handoff layout is
 //! defined in exactly one place.