feat(v4.0.0): block storage, LOAD and LIST (D-19)

- The golden model's host node gets a block storage device: four
  memory-mapped registers (number, address, command, status), 1024-byte
  blocks as 256 cells.  tests/test_node.c.
- capsule/blocks.v4: BLOCK BUFFER UPDATE SAVE-BUFFERS EMPTY-BUFFERS LIST
  LOAD SCR BLK (FORTH-79) and v3's FLUSH THRU and -->, over two buffers.
- The text being interpreted is at the address in (SRC), which QUERY makes
  the terminal's buffer and LOAD a block's; LOAD saves and restores it, so
  blocks nest and the rest of LOAD's line runs afterwards.  WORD makes
  sure a loading block is still in a buffer before it reads.
- In a block, \ skips to the next 64-character line.
- A block number that does not exist, and --> at the terminal, are errors
  with messages (D-18).
- tests/test_host_quit.c: ten transcripts of the v3 binary; nesting three
  deep on two buffers; what reaches the device and when.

v3's LOAD drops the rest of its line, and v3 has no BLK.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/capsule/core.v4

@@ -16,6 +16,7 @@
 \    7 Control structure mismatch     8 Control structures too deep
 \    9 Shift count out of range      10 Protected word
 \   11 Division by zero (Q./)        12 Argument out of range
+\   13 Block out of range            14 No block is being loaded
 \   -1 the word has printed its own message
 
 macro SWAP   over push push drop pop pop   endmacro