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, 14 insertions, 0 deletions

diff --git a/c/libc.s b/c/libc.s
index 2e2516b..5b81dca 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -199,3 +199,17 @@ _reap:
 	ld	c, #22			; SYS_REAP
 	rst	#0x30
 	ret
+
+	.globl _bfill, _bpeek
+	;; void bfill(unsigned char block /*A*/, unsigned char byte /*E*/)
+_bfill:
+	ld	b, a
+	ld	c, #23			; SYS_BFILL
+	rst	#0x30
+	ret
+	;; unsigned char bpeek(unsigned char block /*A*/) -> A
+_bpeek:
+	ld	b, a
+	ld	c, #24			; SYS_BPEEK
+	rst	#0x30
+	ret