ramdisk: the live root grows into half of memory

The live root was a fixed 96 MB image in the kernel: 60 MB of zeros on
the ISO, twice, and room for 60 MB of packages whatever the machine had.

Now the image is only what the root holds, plus slack, measured by
building it once large. Past its end the ramdisk goes on to half of the
memory free at mount, as pages taken the first time each block is
written; an unwritten block reads as zeros and costs nothing. SchFS
keeps no free-space map on disk, so growing a volume is a larger total
before open builds the map: grow_to, set for the ramdisk root.

8 GB machine: a 4 GB root, Xorg installed into it in 77 s with 7.6 GB
still free. The ISO is 172 MB instead of 344.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/core/defs.h

@@ -368,6 +368,7 @@ void            input_mouse(int dx, int dy, int buttons);
 void            ramdiskinit(void);
 void            ramdiskrw(struct buf*);
 int             ramdisk_present(void);
+uint64          ramdisk_blocks(void);
 
 // usb_storage.c
 void            usb_storage_rw(struct buf*);