feat(v4.0.0): a node is handed a line -- ENGINE.md step 1

A v4 node no longer reads its own command line or prints a prompt.  Its
host puts a line of text in the node's input buffer and starts it at
(LINE); the node interprets it and stops at (IDLE), leaving in
(LINE-STATUS) how it ended: completed, an error, or QUIT.  The host says
" ok" or " ERROR" and prompts, as the kernel's REPL does for a v3 VM.  A
line may be 1024 characters, a block, as v3's.  Ruled 2026-10-05
(V3-PARITY.md 1b); design ENGINE.md 3.1.

- quit.v4: (REPL), the node's prompt loop, is gone; (LINE) (IDLE) (DONE)
- image.h/.c: v4_line_begin, v4_line_done, v4_line_status; the node is
  idle at switch-on
- boot.c: v4_boot_line, the one loop the hosted binary, the kernel and the
  capsule loader hand a line with; the code that took " ok" and the prompt
  back out of the node's output is gone
- hosted.c, sk_v4.c: the prompt and the line editing are the host's
- test_host_quit.c: the tests are the node's host; two tests of the old
  80-character prompt line now test a whole line, 1024 and 1025 characters

Verified: make -C v4 test passes at both widths; hosted-check passes on
three ISAs; clean qemu with STARFORTH_V4=1 on amd64, aarch64 and riscv64
passes POST (550 of 550) with the same hashes as hosted, and three lines
typed at each bare-metal prompt through the serial port are answered
correctly (logs/20261005-180922, -181152, -181541).

Still the lone node: kernel_main.c starts it before the fleet tables.

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

v4/capsule/forth.v4

@@ -34,7 +34,7 @@ header DEPTH
 \ ( name -- )   "xxxx: Invalid index", and the line ends with ERROR
 : (INDEX)
   R-CLEAR  (EMIT4)  $6E49203A (EMIT4) $696C6176 (EMIT4) $6E692064 (EMIT4) $786564 (EMIT4) CR
-  2 jump (REPL)
+  2 jump (DONE)
 
 \ ( x1 .. xn-1 -- )   A: n, which is 2 or more.  Set the n-1 values above
 \ the n-th aside.  If there are not that many, the pop that finds the stack
@@ -127,7 +127,7 @@ header 2/ inline        : TWO/    2/ ;
 : (DIV0)
   R-CLEAR  (EMIT4) (EMIT4)  $6944203A (EMIT4) $69736976 (EMIT4) $62206E6F (EMIT4)      \ ": Division b"
   $657A2079 (EMIT4) $6F72 (EMIT4) CR                                  \ "y zero"
-  2 jump (REPL)
+  2 jump (DONE)
 
 \ ( d n -- rem quot )   truncating toward zero: the quotient has the sign of
 \ d xor n, the remainder the sign of d.  This is section 4's SM/REM with its