Stage 2: cooperative VM context switch primitive, proven on all 3 arches (FABRIC-3.md §XXVIII)

Third stage of the preemptive context-switching plan. The real
save/restore switch mechanism now exists -- the first time anything has
ever executed on a VM's own native stack (Stage 1 allocated them,
unused).

New sk_vm_switch_to() (switch.S, one per arch) is an ordinary function
call, not an interrupt -- so unlike Stage 0's trap frame, the ABI already
covers every caller-saved register; only the callee-saved set needs
explicit save/restore (amd64: rbx/rbp/r12-r15, no FP at all since SysV
has no callee-saved XMM; aarch64: x19-x28/x29/x30 + d8-d15; riscv64:
s0-s11/ra + fs0-fs11, FS-gated like Stage 0 but read once and reused for
both halves within one call, since FS is genuine global CPU state, not
part of what's switched). A sibling sk_vm_switch_prime() in the same file
builds the synthetic first-entry frame, kept in assembly so the layout
can never drift out of sync with sk_vm_switch_to() itself.

New switch.c/switch.h: sk_vm_context_switch(from, to) handles first-entry
priming vs. resuming a parked context, and updates registry state (new
VM_STATE_SWITCHED_OUT, distinct from VM_STATE_STOPPED -- STOPPED means no
live frame, this means the opposite). sk_vm_switch_entry() is the minimal
permanent trampoline every freshly-entered VM lands in: no production
behavior defined yet, so it just yields straight back to whoever switched
to it, forever.

Closes the confirmed unguarded-KILL UAF found during planning:
capsule_vm_kill(), mama_word_kill(), and capsule_vm_kill_all_nonmama()
all now refuse (or silently leak rather than free, on the cold-restart
path where arch_cold_reset() wipes everything immediately after anyway)
tearing down a switched-out VM. Side effect found, not built on purpose:
the existing MSG-TICK idle-pump already filters on VM_STATE_LIVE, so it
automatically stopped dispatching into a switched-out VM with zero
changes needed there.

Verified via a temporary SWITCH-TEST probe (boot-triggered, since nothing
can type interactively into a foreground-only QEMU session) that
round-tripped a sentinel through 5 real Hera<->Hermes switches on all 3
architectures: 5/5 rounds, 0 failures, clean continuation to ok>. Probe
fully reverted after capture; kernel_main.c shows zero diff.

Also: Makefile.starkernel's LOADER_EXTRA_SRCS/LOADER_ASM needed the new
files added explicitly (this project's "loader" PE binary is the full
running kernel, not a thin bootstrap stage), and aarch64's switch.S
needed the same #ifndef _WIN32 guard around .hidden that isr.S already
carries (aarch64's loader assembles via clang targeting a PE/COFF
target with no .hidden equivalent) -- caught by a build failure, fixed.

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

include/starkernel/capsule_run.h

@@ -87,9 +87,23 @@ typedef struct {
 typedef enum {
     VM_STATE_EMBRYO = 0,     /* Allocated but not yet born */
     VM_STATE_LIVE,           /* Successfully born, operational */
-    VM_STATE_STOPPED,        /* Suspended — execution state saved */
+    VM_STATE_STOPPED,        /* Suspended — execution state saved. Set by
+                              * the START word after STOP cleanly unwinds
+                              * its C stack back to START's own frame: no
+                              * live native frame remains, safe to free. */
     VM_STATE_STILLBORN,      /* Birth failed */
     VM_STATE_DEAD,           /* Terminated */
+    VM_STATE_SWITCHED_OUT,   /* FABRIC-3.md §XXVIII, Stage 2 (2026-09-13):
+                              * a live saved register/stack context is
+                              * parked on this VM's own native stack
+                              * (SWITCH-TO switched control away mid-
+                              * execution). Deliberately distinct from
+                              * VM_STATE_STOPPED -- that state means "no
+                              * live native frame," this one means the
+                              * opposite. KILL must refuse/defer here: the
+                              * parked frame still points into vm->memory
+                              * and the native stack, both of which a free
+                              * would invalidate out from under it. */
 } VMState;
 
 typedef struct {