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>

Kconfig.kernel

@@ -33,15 +33,6 @@ config SK_PARITY_DEBUG
 	  treatment as every other previously-unwired constant in this
 	  migration.
 
-config HEARTBEAT_DOE_LOG
-	bool "Per-tick heartbeat DoE logging (HEARTBEAT_DOE_LOG)"
-	default y
-	help
-	  Emits per-heartbeat-tick DoE observation records used by the
-	  bare-metal Design-of-Experiments campaigns
-	  (experiments/bare_metal/). On by default; the DoE tooling expects
-	  this log to be present.
-
 config STADIUM_MAX_VM_COUNT
 	int "Outer Stadium VM population bound (STADIUM_MAX_VM_COUNT)"
 	default 4