Cursor indicator + HB-ON/HB-OFF runtime DoE instrumentation toggle

Cursor (Captain Bob: "the only thing we need is a cursor"):
vt100_draw_cursor() draws a solid block at the terminal's current
position, called from repl.c after the prompt prints and after every
keystroke/backspace. vt100_erase_cursor() cleans up the one gap a static
cursor has -- Enter/newline moves away from the cursor cell without a
character draw ever overwriting it, which left a stray block behind
until this fix.

HB-ON/HB-OFF (Captain Bob: run a program with or without instrumentation
without rebuilding):
Converted per-tick DoE logging from a build-time flag (HEARTBEAT_DOE_LOG)
to a runtime one. doe_log_tick_row() now self-gates on g_doe_log_enabled
(default 1, matching the old default) instead of being compiled out
entirely; the call site in vm_runtime.c is unconditional. Two new FORTH
words, HB-ON and HB-OFF, flip the flag live. Removed the now-dead
HEARTBEAT_DOE_LOG plumbing: the Kconfig symbol, and the -D forwarding in
both LOADER_CFLAGS and KERNEL_CFLAGS.

Verified: three-arch clean QEMU boot + logs; dictionary word count 466
(463 baseline + ALT+TAB + HB-ON + HB-OFF, exactly the three words added
across this session); amd64 screendump confirms the cursor renders
correctly after real interactive typing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/word_registry.c

@@ -77,6 +77,9 @@
 #include "word_source/include/keyboard_words.h"
 #include "word_source/include/ttf_words.h"
 #include "word_source/include/scroll_words.h"
+#ifdef __STARKERNEL__
+#include "starkernel/doe_log.h"
+#endif
 
 /**
  * @brief Registers a single FORTH word in the virtual machine
@@ -136,6 +139,9 @@ void register_forth79_words(VM *vm) {
     register_keyboard_words(vm); /* Module 29: Console fabric -- raw keyboard scancode diagnostic */
     register_ttf_words(vm); /* Module 30: TrueType text entry point */
     register_scroll_words(vm); /* Module 31: REPL scrollback (FABRIC.md 4.4q) */
+#ifdef __STARKERNEL__
+    register_doe_log_words(vm); /* Module 32: DoE instrumentation runtime toggle (HB-ON/HB-OFF) */
+#endif
 
     log_message(LOG_INFO, "FORTH-79 Standard word set registration complete");
 }