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/starkernel/vm/vm_runtime.c

@@ -47,9 +47,7 @@
 #include "../include/inference_engine.h"
 #include "../include/physics_metadata.h"
 #include "../include/physics_hotwords_cache.h"
-#if defined(HEARTBEAT_DOE_LOG) && HEARTBEAT_DOE_LOG
 #include "starkernel/doe_log.h"
-#endif
 #include "../include/ssm_jacquard.h"
 #include "vm_internal.h"
 #include "starkernel/capsule_vm_physics.h"
@@ -516,9 +514,9 @@ void vm_heartbeat_run_cycle(VM *vm)
 #if defined(HEARTBEAT_CSV_ENABLED) && HEARTBEAT_CSV_ENABLED
     heartbeat_emit_tick_row(vm, &tick_snapshot);
 #endif
-#if defined(HEARTBEAT_DOE_LOG) && HEARTBEAT_DOE_LOG
+    /* Always compiled in; g_doe_log_enabled (default 1) is the runtime
+     * gate now, toggled live by HB-ON/HB-OFF -- see doe_log.h. */
     doe_log_tick_row(vm, &tick_snapshot);
-#endif
 }
 
 #if HEARTBEAT_THREAD_ENABLED