fix(v4): the kernel's fatal stops halt, interrupts off, instead of spinning (#177)

sk_v4_run's three stops -- "not started", "did not let go of POST's
blocks", "stopped on a fault" -- ended in for (;;) { }: a core spinning at
100%, the thing #151 removed from the prompt. Each now calls sk_v4_stop()
(kernel/src/v4/sk_v4_stop.c): interrupts off, then arch_halt() for ever. Each
still says why first. sk_v4_stop.c added to the loader's and the kernel's v4
sources.

kernel/tests/test_sk_v4_stop.c, with arch_halt and arch_disable_interrupts
faked (the halt jumps back out after three calls; an alarm fails a spin):
against the stop as it was -- "spun for 2 s without halting"; now -- 2
checks, 0 failures: halts in a loop, interrupts off before the first halt.
Built for amd64, aarch64 and riscv64. #177's "seen on the three
architectures": no boot reaches these stops, so it is shown on the host only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017RzZgrf7V7Zk2iczBXDRbm

kernel/include/starkernel/v4/sk_v4_stop.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LicenseRef-Starship-1.0
+ * Copyright (c) 2023-2026 Robert A. James
+ */
+/* sk_v4_stop.h -- the v4 kernel's fatal stop (#177): what sk_v4_run does when the system cannot
+ * go on ("not started", "did not let go of POST's blocks", "stopped on a fault"). Never returns.
+ */
+#ifndef STARKERNEL_V4_SK_V4_STOP_H
+#define STARKERNEL_V4_SK_V4_STOP_H
+
+void sk_v4_stop(void) __attribute__((noreturn));
+
+#endif