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.

tools/mkinitrd.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Build-time initrd packer. Concatenates user binaries into one image the
+bootloader ferries to the kernel.
+
+Usage: mkinitrd.py <out.img> [<name> <file>]...
+
+Image layout (little-endian), mirroring src/user/proto/initrd.zig:
+    Header   : magic u32 ("DNRD"=0x444E5244), count u32
+    Entry*N  : name [32]u8 (NUL-padded), offset u64, len u64
+    blobs    : each entry's file bytes at its offset
+"""
+import struct
+import sys
+
+MAGIC = 0x444E5244
+HEADER = struct.Struct("<II")     # magic, count
+ENTRY = struct.Struct("<32sQQ")   # name[32], offset, len
+
+
+def main() -> int:
+    out_path = sys.argv[1]
+    rest = sys.argv[2:]
+    if len(rest) % 2 != 0:
+        sys.stderr.write("usage: mkinitrd.py <out.img> [<name> <file>]...\n")
+        return 2
+    items = [(rest[i], rest[i + 1]) for i in range(0, len(rest), 2)]
+
+    table_end = HEADER.size + len(items) * ENTRY.size
+    entries = b""
+    blobs = []
+    off = table_end
+    for name, path in items:
+        with open(path, "rb") as f:
+            data = f.read()
+        entries += ENTRY.pack(name.encode()[:31], off, len(data))
+        blobs.append(data)
+        off += len(data)
+
+    with open(out_path, "wb") as f:
+        f.write(HEADER.pack(MAGIC, len(items)))
+        f.write(entries)
+        for b in blobs:
+            f.write(b)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())