Stage 3: timer-driven preemptive switching, live on all 3 arches (FABRIC-3.md §XXVIII)

Fourth stage of the preemptive context-switching plan, and the biggest.
LithosAnanke now genuinely, continuously preempts between Hera, Hermes,
and Artemis -- timer-driven, running live for the entire remainder of
every boot once the Tripod fleet registers, not a bounded probe.

A real design fork was resolved before writing code: the naive approach
(the timer ISR calling Stage 2's sk_vm_context_switch() directly) is
broken -- Stage 0's trap frame lives on whatever stack was active at
interrupt time, and jumping to a different stack via Stage 2's own
independent swap mid-handler would abandon that trap frame unresumed,
guaranteed corruption on the first tick. Chose the safer of two named
options: the ISR only ever sets a flag and returns completely normally
through its own full epilogue; the actual switch happens moments later,
via Stage 2's already-proven mechanism, at a safe cooperative checkpoint
on the mainline (execute_colon_word()'s per-word dispatch loop, checked
on literally every word, not throttled to the existing 256-word
heartbeat-tuning cadence) -- confirmed with the user that word-level
granularity is fine-grained enough given the eventual Zynq FPGA target
where a word is a mnemonic.

New capsule_vm_switch_signal.c/.h: a purpose-built run-readiness signal,
deliberately separate from capsule_vm_physics.c's execution-heat engine
(that one's own header documents itself as never touched from interrupt
context, by design). Slot table sized with headroom (8) rather than
hardcoded to today's 3 participants, so extending participation later is
another register() call, not a redesign -- per direct request to leave
room for swapping the participant set. Simple linear accumulate-then-
threshold for this first cut; a fancier law can replace it later without
touching the mechanism around it. heartbeat_tick() gains its one
deliberate, documented amendment to this file's own top-half/bottom-half
discipline -- the first time this codebase reaches into VM-scheduling
state from real ISR context.

Registration happens only after all three VMs are fully born, right
before the REPL starts -- no critical-section protection yet against
being switched away mid-birth-setup.

Known, flagged rough edge (not reconciled this pass): MSG-TICK's own
idle-pump and this new mechanism can still independently move control
between the same VMs; not observed to interact badly in verification,
but not fully unified either.

Verified interactively at the console on all 3 architectures with
continuous background preemption running throughout -- amd64 computed
`1 1 + .` -> 2, aarch64 computed `1 1 + dup DUP * . CR` -> 4, both
correct, REPL fully responsive, zero fault indicators over sustained
runtime.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016UNhH1mhi52i6Qihh7ZV5S

src/starkernel/vm/vm_core.c

@@ -60,6 +60,9 @@
 #include "starkernel/console.h"  /* g_sk_fault_word */
 #include "platform_alloc.h"      /* sf_free for call_stack */
 #include "starkernel/vm/stadium_words.h" /* item 4.1: word patrons on the Stadium */
+#include "starkernel/capsule_vm_switch_signal.h" /* FABRIC-3.md §XXVIII Stage 3 */
+#include "starkernel/vm/switch.h" /* FABRIC-3.md §XXVIII Stage 2 -- sk_vm_context_switch() */
+#include "starkernel/capsule_birth.h" /* capsule_vm_registry_get() */
 #endif
 #include "word_source/include/vocabulary_words.h"
 #include "vm_internal.h"
@@ -880,6 +883,30 @@ void execute_colon_word(VM* vm)
             vm->heartbeat.check_counter = 0;
         }
 
+#ifdef __STARKERNEL__
+        /* FABRIC-3.md §XXVIII, Stage 3 (2026-09-13): the cooperative
+         * preemption checkpoint. Deliberately unconditional (every word,
+         * not throttled like the heartbeat cycle above) -- the check
+         * itself is a single flag read, cheap regardless of frequency,
+         * and "a word is a mnemonic" (this project's own eventual FPGA
+         * framing) is the granularity preemption should approximate here.
+         * Only ever acts when this word's own execution left the VM in a
+         * clean, resumable state (past the error/abort/exit_colon checks
+         * below would be too late -- this runs before them, deliberately,
+         * so a switch never happens mid-unwind). */
+        {
+            VMUuid target_id = sk_vm_switch_signal_take_pending();
+            if (!vm_uuid_equal(target_id, vm_uuid_none())) {
+                VMRegistryEntry target_entry;
+                if (capsule_vm_registry_get(target_id, &target_entry) == 0 &&
+                    target_entry.vm_ptr && target_entry.vm_ptr != (void *)vm) {
+                    sk_vm_context_switch(vm, (VM *)target_entry.vm_ptr);
+                    /* Resumes here once something later switches back. */
+                }
+            }
+        }
+#endif
+
         if (vm->error) { vm->ecw_nesting--; return; }
 
         /* ABORT clears stacks and unwinds to QUIT: leave the flag set so