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

capsules/BLOCK_MAP.md

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