feat(tools): add mktar ramdisk builder and sample fsroot content

fsroot/etc/motd.txt

@@ -0,0 +1,5 @@
+frostbyte ramdisk file
+frostbyte-ustar-read-ok
+This text lives inside a USTAR (tar) image built by tools/mktar and handed to
+the kernel as a Multiboot2 module. Seeing this line on the serial port proves
+the kernel parsed the tar header and read the file body through the VFS.

fsroot/etc/readme.txt

@@ -0,0 +1,8 @@
+frostbyte filesystem layout
+
+/bin    ring-3 ELF programs loaded by the kernel at boot
+/etc    text files packed into the ramdisk
+/home   more sample text
+/tmp    writable in-RAM tmpfs, empty at boot
+
+Try from the shell: ls /etc, cat /etc/motd.txt, mkdir /tmp/x, echo hi >/tmp/x/a.

fsroot/home/notes.txt

@@ -0,0 +1,2 @@
+nested directory entry in the ramdisk.
+the kernel created /home while walking the tar path home/notes.txt.