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/input.v4

@@ -13,13 +13,22 @@
 \            EXPECT or QUERY read
 \   WBUF     byte address of WORD's buffer, 257 bytes
 \   (P)      word address of eight cells of scratch for this file
+\   (SRC)    word address of the variable: the byte address of the text being
+\            interpreted.  QUERY makes it TIB; LOAD (blocks.v4) makes it a
+\            block buffer.  >IN is an offset from it and SPAN its length.
+\   (SRC-HOOK)   word address of a variable: 0, or the address of a word
+\            that WORD calls before it looks at the text, to make sure (SRC)
+\            is right.  LOAD puts one there: a block buffer can be given to
+\            another block between one word of the text and the next.
+\   BLK      word address of the variable: the block being interpreted, or
+\            0 for the terminal (FORTH-79)
 \ TIB >IN SPAN and BL are in-line: written in a definition they are literals.
 
 macro BL  32  endmacro
 
 \ ( -- baddr u )   the input buffer and how much of it is in use
 header SOURCE
-: SOURCE   TIB  SPAN a! @  -if P  drop 0  P: ;
+: SOURCE   (SRC) a! @  SPAN a! @  -if P  drop 0  P: ;
 
 \ ( baddr n -- )   FORTH-79: characters from the terminal are stored from
 \ baddr upward until a new-line or until n have been received; the new-line
@@ -49,7 +58,7 @@ header EXPECT
 
 \ ( -- )   FORTH-79: up to 80 characters, or a line, into TIB; >IN to 0.
 header QUERY
-: QUERY   TIB 80 EXPECT  0 >IN a! ! ;
+: QUERY   TIB (SRC) a! !  TIB 80 EXPECT  0 >IN a! ! ;
 
 \ ---- WORD ------------------------------------------------------------------
 \ WORD is called from deep inside the interpreter and compiler, with whatever
@@ -64,7 +73,7 @@ header QUERY
 
 \ In line, not called: WORD may have only a few return entries to spare.
 macro (LEFT)   (P)+6 a! @  (P)+1 a! @  -            endmacro   \ ( -- d )  d < 0 while inside the text
-macro (CH)     (P)+6 a! @ TIB + C@  (P) a! @  xor   endmacro   \ ( -- x )  x = 0 at a delimiter
+macro (CH)     (P)+6 a! @ (SRC) a! @ + C@  (P) a! @  xor   endmacro   \ ( -- x )  x = 0 at a delimiter
 macro (STEP)   (P)+6 a! @ 1 + !                     endmacro
 
 \ ( c -- baddr )   FORTH-79: characters are taken from TIB until the
@@ -78,6 +87,10 @@ macro (STEP)   (P)+6 a! @ 1 + !                     endmacro
 \ than if it were written in one piece.
 : (WORD-SET) ( c -- )
   255 and (P) a! !
+  (SRC-HOOK) a! @ if NONE  push ex           \ call it: ex comes back to the next word
+  jump HOOKED
+  NONE: drop
+  HOOKED:
   SPAN a! @  -if A drop 0 A:  (P)+1 a! !
   >IN a! @   -if B drop 0 B:  (P)+6 a! ! ;
 
@@ -91,7 +104,7 @@ macro (STEP)   (P)+6 a! @ 1 + !                     endmacro
   (P)+2 a! @ WBUF C!
   \ copy it, last character first
   CP: (P)+3 a! @ if COPIED  -1 + dup !       \ k
-      dup (P)+7 a! @ + TIB + C@              \ k c
+      dup (P)+7 a! @ + (SRC) a! @ + C@       \ k c
       SWAP WBUF+1 + C!
       jump CP
   COPIED: drop
@@ -190,4 +203,7 @@ header NUMBER
 header ( immediate
 : PAREN      41 WORD drop ;
 header \ immediate
-: BACKSLASH  SPAN a! @  >IN a! ! ;
+\ In a block a line is 64 characters: skip to the start of the next.
+: BACKSLASH
+  BLK a! @ if TERM  drop  >IN a! @ -64 and 64 + ! ;
+  TERM: drop  SPAN a! @  >IN a! ! ;