Add HEARTBEAT-TICKS@: read-only heartbeat tick counter accessor

The adaptive heartbeat tick counter (vm->heartbeat.tick_count) is the
project's sole canonical clock for timing measurements -- host wall-clock
is not a valid substitute. Exposes it read-only so DoE/overhead campaigns
can measure elapsed ticks instead of wall-clock deltas.

Verified: three-arch QEMU acceptance (amd64/aarch64/riscv64), POST
1012/0/0 on each, HEARTBEAT-TICKS@ live-tested returning a real non-zero
count on all three.

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

capsules/BLOCK_MAP.md

@@ -1,5 +1,5 @@
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-<!-- Generated by mkcapsule --manifest  2026-08-20T14:53:07Z -->
+<!-- Generated by mkcapsule --manifest  2026-08-21T03:09:23Z -->
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 <!-- Hand-written justifications and immutability notes live  -->
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src/word_source/starforth_words.c

@@ -793,6 +793,20 @@ static void starforth_word_zuse_authenticate(VM *vm)
     vm->zuse_session = 1;
 }
 
+/**
+ * @brief Read-only accessor for the canonical heartbeat tick counter
+ *
+ * Stack effect: ( -- n )
+ * Pushes vm->heartbeat.tick_count -- the one clock this project's timing
+ * measurements are supposed to read, not host wall-clock. Read-only: no
+ * corresponding store word exists or should exist.
+ * @param vm Pointer to the VM instance
+ */
+static void starforth_word_heartbeat_ticks(VM* vm)
+{
+    vm_push(vm, (cell_t)vm->heartbeat.tick_count);
+}
+
 /**
  * @brief Register StarForth vocabulary words with the VM
  *
@@ -812,6 +826,7 @@ void register_starforth_words(VM* vm)
     register_word(vm, "RANDOM", starforth_word_random);
     register_word(vm, "WAIT", starforth_word_wait);
     register_word(vm, "ZUSE-AUTHENTICATE", starforth_word_zuse_authenticate);
+    register_word(vm, "HEARTBEAT-TICKS@", starforth_word_heartbeat_ticks);
 
     vm_bootstrap_root_vocabulary(vm, "STARFORTH");
     STARFORTH_CHECK_ARENA("register_starforth_words:post-root");
@@ -829,6 +844,7 @@ void register_starforth_words(VM* vm)
     register_word(vm, "RANDOM", starforth_word_random);
     register_word(vm, "WAIT", starforth_word_wait);
     register_word(vm, "ZUSE-AUTHENTICATE", starforth_word_zuse_authenticate);
+    register_word(vm, "HEARTBEAT-TICKS@", starforth_word_heartbeat_ticks);
 
     vocabulary_word_forth(vm);
     vocabulary_word_definitions(vm);