fs rewrite stage 1: persistent block device (bounce buffer, cart SRAM)

- blk.asm: read_block/write_block move 256-byte blocks between battery-backed
  cart-RAM banks (8-11, the 'disk') and a WRAM bounce buffer. Banking is hidden
  in those two routines; they inline the 'restore my bank' step so they never
  touch the stack while the disk bank is mapped over the process's $A000 window
  (the stack lives there too -- a call/ret would rug-pull it).
- format-on-first-boot: superblock magic in block 0; otherwise the disk persists.
- cart RAM bumped to 128 KiB (-r 4); process banks 0-7, disk banks 8-11.
- debug tool 'blk fill/peek' + syscalls to validate round-trip and persistence.
- verified: round-trip incl. cross-bank (block 100 -> bank 11); block survives a
  reboot via .sav; magic intact (no reformat on 2nd boot).
- old WRAM 8-slot FS still backs the tools until stages 2-3.

1 files changed, 2 insertions, 0 deletions

diff --git a/c/gbos.h b/c/gbos.h
index f76d1e0..03a91de 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -43,4 +43,6 @@ unsigned char psget(unsigned char slot, unsigned char *buf); /* {pid,state,prog}
 void          progname(unsigned char id, char *buf);         /* id -> name     */
 void          yield(void);
 unsigned char reap(void);                   /* reap a finished bg job, or 0   */
+void          bfill(unsigned char block, unsigned char byte); /* [debug] disk */
+unsigned char bpeek(unsigned char block);                     /* [debug] disk */
 #endif