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

include/starkernel/capsule_vm_switch_signal.h

@@ -0,0 +1,79 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+*/
+
+/**
+ * capsule_vm_switch_signal.h - New, purpose-built "who runs next" signal
+ * for preemptive context switching (FABRIC-3.md §XXVIII, Stage 3,
+ * 2026-09-13).
+ *
+ * Deliberately NOT a repurposing of capsule_vm_physics.c's execution-heat
+ * engine -- that measures word-level dispatch fairness over millions of
+ * executions on a different timescale, and its own header explicitly
+ * documents it as never touched from interrupt context (unlocked, by
+ * design). This is a different physical quantity: instant-by-instant
+ * run-readiness, consulted from real ISR context (heartbeat_tick()) every
+ * timer tick.
+ *
+ * Concurrency discipline mirrors heartbeat.c's own §21.1-sanctioned
+ * pattern for heartbeat_next_period_ns(): single-writer-ISR (tick()) /
+ * single-reader-mainline (take_pending(), called from the cooperative
+ * checkpoint in execute_colon_word()), no lock, because nothing on this
+ * single hart is concurrent with the ISR while it runs.
+ *
+ * NOT truly interrupt-driven register/stack swapping (that was
+ * considered and rejected for this stage -- see FABRIC-3.md §XXVIII
+ * Stage 3 for why): the ISR only ever sets a flag. The actual switch
+ * (Stage 2's already-proven sk_vm_context_switch()) happens later, at a
+ * safe cooperative checkpoint on the mainline, once per word dispatch.
+ *
+ * Slot table is sized with headroom, not hardcoded to exactly today's 3
+ * participants (Hera/Hermes/Artemis) -- extending participation later
+ * (Stage 4+) is another sk_vm_switch_signal_register() call, not a
+ * redesign.
+ */
+
+#ifndef STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H
+#define STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H
+
+#ifdef __STARKERNEL__
+
+#include "starkernel/vm_uuid.h"
+
+/* Register a VM as a switch-signal participant. Returns its slot index,
+ * or -1 if the slot table is full. Call once per participating VM,
+ * after that VM is fully born (never mid-birth -- this stage has no
+ * critical-section protection against being switched away mid-setup). */
+int sk_vm_switch_signal_register(VMUuid vm_id);
+
+/* Called from heartbeat_tick() (ISR context) every timer tick. Cheap:
+ * iterates only the registered slots (bounded, small). */
+void sk_vm_switch_signal_tick(void);
+
+/* Called from the cooperative checkpoint (execute_colon_word(), mainline,
+ * once per word dispatch). Returns the VMUuid of a VM that should now be
+ * switched to, or vm_uuid_none() if nothing is pending. Clears the
+ * pending flag as a side effect -- call at most once per checkpoint. */
+VMUuid sk_vm_switch_signal_take_pending(void);
+
+#endif /* __STARKERNEL__ */
+
+#endif /* STARKERNEL_CAPSULE_VM_SWITCH_SIGNAL_H */