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>

src/starkernel/vm/switch.c

@@ -49,8 +49,9 @@ extern uint64_t sk_vm_switch_prime(uint64_t stack_top, void *entry);
  * native stack belongs to. Read exactly once, synchronously, at the very
  * top of sk_vm_switch_entry() before any other first-entry could occur.
  * Safe to keep as a plain global (unlike the old g_switch_back_to this
- * file used to also carry -- see native_switch_back_to's own doc comment
- * in vm.h for why that one had to move to per-VM state): a VM's identity
+ * file used to also carry, which moved to per-VM native_switch_back_to and
+ * was then removed at FABRIC-3.8.md Stage I once the trampoline stopped
+ * yielding to "whoever switched in"): a VM's identity
  * never changes across the lifetime of its own trampoline instance, only
  * who most recently switched into it does. */
 static VM *g_switch_entry_vm;
@@ -236,11 +237,6 @@ int sk_vm_context_switch(VM *from, VM *to) {
         return -1;
     }
 
-    /* Recorded unconditionally (first-ever entry or a resume) so whoever
-     * is parked at `to`'s native stack -- most often this placeholder
-     * trampoline -- always knows who most recently switched into it. */
-    to->native_switch_back_to = from;
-
     if (to->native_stack_saved_sp == 0) {
         /* First-ever entry: synthesize the initial frame. */
         g_switch_entry_vm = to;