feat: SKEL skeleton instrumentation - designed-but-unimplemented paths panic

- hypercall.c: 5 Phase 1.3 stubs -> HCALL-E01..E05 panic (no NOSYS return)
- drr_stub.c: snapshot alloc fail -> DRR-E01; new skeletons DRR-E02 root key
  seal, DRR-E03 task kill crypto erase, DRR-E04/E05 segment/system rollback,
  DRR-E06/E07 driver recovery ops dispatch
- sched.c: enable fail -> SCH-E02 panic (no silent demo skip)
- dkm/kernel_api.c: D1 hot-unload DKM-E01, D2 bare code loader DKM-E02,
  SIPI AP wake DKM-E03 skeletons
- virtio_net tx/rx stubs -> VNET-E01/E02; ath9k tx stub -> ATH-E01;
  xhci port reset -> XHC-E01; nvme MSI/MSI-X fail -> NVME-E01..E03 (no
  silent polling downgrade); e1000 MAC fallback -> E1K-E01
- pe_shim unimpl API -> PE-E01; PE ordinal import -> PE-E02
- kernel/main.c: lib sync / vscode install fail -> LNX-E01/E02
- NEW brand/brand.c (E1 boot anim BRAND-E01, E2 time egg BRAND-E02) and
  pkg/pkg.c (pacman skeleton PKG-E01..03), wired into UTSM MAKEFILE
- Deliberate design-internal fallbacks kept and documented (VMX skip,
  bzImage skip, KDE fallback, boot-module fallback, apic->PIC route)

CODE/UTSM/sched/sched.c

@@ -17,6 +17,7 @@
 #include <utsm/sched.h>
 #include <utsm/sched_ext.h>
 #include <utsm/log.h>
+#include <utsm/panic.h>
 #include <utsm/arena.h>
 #include <utsm/config.h>
 #include <utsm/process.h>
@@ -630,7 +631,9 @@ void utsm_sched_demo_window(void) {
     log_hex64("[SCHED] demo tasks=", (u64)((a >= 0) + (b >= 0) + (c >= 0)));
 
     if (utsm_sched_enable(0) != 0) {
-        log_warn("[SCHED] enable failed, demo window skipped");
+        /* 严格错误策略插桩：enable 失败不允许静默跳过演示窗口继续启动 */
+        panic_full("SCH-E02 SCHED ENABLE FAILED",
+                   "utsm_sched_enable failed; skipping demo window is forbidden", 0);
         return;
     }