Stage I: strip dead code -- pump drain loop, SWITCH-MARK-WORK, global interpret depth, native_switch_back_to (FABRIC-3.8 §4 row I)

Each shown dead three ways (FABRIC-3.5 §XXII): the pump loop by its
condition, by construction (birth cap == switch table capacity, every birth
registers) and at run time (probe silent through boot, G interleave, an
8-worker DoE trial and the relay); SWITCH-MARK-WORK by callers, capsules
(source and baked), experiments/tools/logs; the depth counter and back_to
field by having no readers.

Kept: Hera's own pump drain (last timer-paced delivery, Hera only) and the
always-0 readiness CSV columns (a DoE schema call). Acceptance on amd64/
aarch64/riscv64: G interleave unchanged, cfg-0 trial fail=0 2/2 dead,
relay 3, idle growth 0; dict_hash set changed by the removed word
(word_count 531) and identical across ISAs. FABRIC-3.8 stages A-I done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

include/vm.h

@@ -659,17 +659,6 @@ typedef struct VM
                                          *   entered (no context parked yet); non-zero only
                                          *   while VMRegistryEntry.state ==
                                          *   VM_STATE_SWITCHED_OUT for this VM. */
-    struct VM *native_switch_back_to;  /**< Stage 3 correction (FABRIC-3.md §XXVIII,
-                                         *   2026-09-13): who most recently switched INTO
-                                         *   this VM. Updated by sk_vm_context_switch() on
-                                         *   every switch-in (first-ever or resumed), so the
-                                         *   placeholder trampoline (sk_vm_switch_entry())
-                                         *   always yields back to whoever actually switched
-                                         *   in, not a stale value captured once at this
-                                         *   VM's very first entry -- a single global there
-                                         *   let a later switch-in from a different VM get
-                                         *   silently bounced to the original caller instead
-                                         *   of itself. */
     VMFrameStack frames;               /**< FABRIC-3.8.md §3.12 (Stage FR, 2026-09-23):
                                          *   this VM's execution frame stack -- the nested
                                          *   levels running on its own native stack, each