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/tests/Makefile

@@ -19,7 +19,8 @@ CC     ?= cc
 CFLAGS := -std=c99 -Wall -Wextra -Werror -O2 -g -I$(HERE) -I$(ROOT)/kernel/include \
           -I$(ROOT)/kernel/include/starkernel -I$(ROOT)/v3/include
 
-TESTS := test_gpt test_usb_config
+TESTS := test_gpt test_usb_config test_sk_v4_stop
+SRC_test_sk_v4_stop := $(ROOT)/kernel/src/v4/sk_v4_stop.c
 SRC_test_gpt := $(ROOT)/kernel/src/gpt.c
 SRC_test_usb_config := $(ROOT)/kernel/src/usb/usb_config.c