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, 13 insertions, 0 deletions

diff --git a/c/blk.c b/c/blk.c
new file mode 100644
index 0000000..adf2a58
--- /dev/null
+++ b/c/blk.c
@@ -0,0 +1,13 @@
+#include "gbos.h"
+/* blk: exercise the block device.  blk fill <n> <v>   |   blk peek <n> */
+void main(void) {
+    char *argv[4];
+    unsigned char argc = argv_parse(argv, 4);
+    if (argc >= 3 && argv[0][0] == 'f') {
+        bfill(atou(argv[1]), atou(argv[2]));
+    } else if (argc >= 2 && argv[0][0] == 'p') {
+        putu(bpeek(atou(argv[1]))); nl();
+    } else {
+        puts("usage: blk fill <n> <v> | blk peek <n>"); nl();
+    }
+}