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.

sbin/vfs.zig

@@ -0,0 +1,18 @@
+//! /sbin/vfs — the user-space VFS server. Shipped in the initrd and spawned as a
+//! ring-3 process. This is a placeholder that only heartbeats, proving the initrd
+//! pipeline ships and spawns it; the real path namespace + IPC dispatch loop
+//! (open/read/write/stat forwarded to driver processes) is built in M9.
+
+const rt = @import("rt");
+
+pub fn main() void {
+    while (true) {
+        _ = rt.sys.write("vfs: alive\n");
+        rt.sys.sleep(1000);
+    }
+}
+
+pub const panic = rt.panic;
+comptime {
+    _ = &rt.start._start;
+}