M8: initrd handoff + multi-binary shipping

Ship more than one user binary: the bootloader now ferries an initrd bundle
(the VFS server + future drivers) alongside sbin/init, and the kernel unpacks
and spawns each program.

- src/user/proto/initrd.zig: the container format (Header{magic,count} +
  Entry{name[32],offset,len} + blobs) with a validating Reader, shared by the
  kernel and the packer.
- tools/mkinitrd.py: host-side packer (Python — trivial format, and sidesteps
  the reworked Zig 0.16 std fs/args API). build.zig runs it on the built user
  binaries via addSystemCommand and installs initrd.img to zig-out/bin + the
  ESP root.
- BootInfo gains initrd_base/initrd_len; efi.zig loadInit refactored into a
  shared loadFile, and loadInitrd ferries \initrd.img into surviving
  LoaderData like init.
- main.zig startInitrdBinaries(): parse the image, spawn every entry (kernel-
  spawns-all for now; init takes over via sys_spawn later).
- sbin/vfs.zig: a heartbeat stub (the real VFS server is M9), packed into the
  initrd to prove the pipeline.
- New `initrd` test: parse + spawn + confirm the vfs stub reaches ring 3 and
  heartbeats. Harness ships initrd.img on the ESP. Suite 30/30.

test/qemu_test.py

@@ -57,8 +57,9 @@ ARCHES = {
         ],
         "efi_app": ("EFI/BOOT/BOOTX64.efi", "BOOTX64.efi"),  # (dest in ESP, name in zig-out/bin)
         "kernel": ("kernel", "kernel"),
-        # Further files shipped on the ESP: the init user program.
-        "extra": [("sbin/init", "init")],
+        # Further files shipped on the ESP: the init user program and the initrd
+        # (VFS server + drivers), both copied from zig-out/bin.
+        "extra": [("sbin/init", "init"), ("initrd.img", "initrd.img")],
         # Built as a function so we can splice in per-run paths.
         "qemu_args": lambda a, esp, vars_fd, serial: [
             "-machine", "q35", "-m", "128M",
@@ -177,6 +178,11 @@ CASES = [
      "smp":    4,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The initrd: the loader ferries a bundle of user binaries; the kernel parses
+    # it and spawns each as a ring-3 process (here the VFS-server stub heartbeats).
+    {"name": "initrd",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The ACPI power path succeeds by QEMU *exiting* (S5 off / reset), so match the
     # pre-transition marker; the FAIL line only appears if the transition didn't take.
     {"name": "poweroff",