Stage MC-0a: per-core block on one core -- current VM moves behind the per-CPU register (FABRIC-3.10.8 §3.1)

- New sk_percpu_t { self, core_id, current_vm }, reached via GS base
  (amd64, %gs:0), TPIDR_EL1 (aarch64), tp (riscv64). Boot core's block is
  one static object installed right after arch_early_init(); secondary
  cores' blocks come with MC-1.
- switch.c's global g_switch_current_vm is gone; current VM is
  sk_percpu()->current_vm, re-read after resume in the context switch.
- Probe (removed): normal path and every heartbeat ISR found core 0's
  block on all three ISAs (ISR ok 889/1877/1625, bad 0).
- Standard sequence on amd64/aarch64/riscv64: hashes unchanged from O-C,
  G A=40 B=40, MINT/WIREBIND/USE ok, conservation true, 0 idle growth.

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

src/starkernel/hal/percpu.c

@@ -0,0 +1,40 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+*/
+
+/**
+ * percpu.c - The boot core's per-core block (FABRIC-3.10.8 Stage MC-0a).
+ * See starkernel/percpu.h.
+ */
+
+#include "starkernel/percpu.h"
+
+/* The boot core's block: one object, not a table sized to a core count.
+ * Secondary cores get theirs as they come online (MC-1). */
+static sk_percpu_t g_percpu_boot;
+
+void sk_percpu_boot_init(void)
+{
+    g_percpu_boot.self       = &g_percpu_boot;
+    g_percpu_boot.core_id    = 0;
+    g_percpu_boot.current_vm = (struct VM *)0;
+    sk_percpu_install(&g_percpu_boot);
+}