fix(v4): a controller's drive slots are allocated once, never replaced (#178)

sk_v4_idle allocated a controller's slot array again whenever max_slots + 1
was more than it held -- the old array dropped, neither copied nor freed --
while the identity code holds pointers into it (g_offered, g_spare,
g_pending_real, g_real_drive, g_zuse_dev). Its two slot loops then ran to
max_slots whatever the array held.

sk_v4_slots_get() (kernel/src/v4/sk_v4_slots.c): allocated once, zeroed, at the
size first asked for; never replaced; the slots that may be used are those it
has. sk_v4_idle uses it, and its loops stop at the array's end.

kernel/tests/test_sk_v4_slots.c, kmalloc_aligned faked, seen to fail first
against the allocation as it was (4 failures: replaced when more were asked
for, 17 said usable where 9 existed, a held pointer left behind, an allocation
for 0); now 6 checks, 0 failures. Built for amd64, aarch64, riscv64; latent
today (max_slots does not grow), so the boots show no change.

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 test_sk_v4_stop
+TESTS := test_gpt test_usb_config test_sk_v4_stop test_sk_v4_slots
+SRC_test_sk_v4_slots := $(ROOT)/kernel/src/v4/sk_v4_slots.c
 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

kernel/tests/test_sk_v4_slots.c

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: LicenseRef-Starship-1.0
+ * Copyright (c) 2023-2026 Robert A. James
+ */
+/* test_sk_v4_slots.c -- kernel/src/v4/sk_v4_slots.c on the host (#178): a controller's drive slots
+ * are allocated once and never replaced, so the pointers the identity code keeps into them (the
+ * offered stick, the spare, Zuse's device) stay good. kmalloc_aligned is faked.
+ */
+#define _POSIX_C_SOURCE 200809L
+#include "harness.h"
+#include "starkernel/v4/sk_v4_slots.h"
+#include <stdlib.h>
+#include <string.h>
+
+static unsigned allocs;
+void *kmalloc_aligned(size_t size, size_t align) { void *p = 0; allocs++; return posix_memalign(&p, align, size) ? 0 : p; }
+
+typedef struct { int attached; char name[16]; } drive;   /* a slot, as sk_v4_drive_t is to the identity code */
+
+int main(void)
+{
+    void *slots = 0;
+    uint32_t count = 0, n;
+    drive *d, *held;
+
+    n = sk_v4_slots_get(&slots, &count, 9u, sizeof(drive));
+    d = (drive *)slots;
+    CHECK(n == 9u && d && allocs == 1u && d[3].attached == 0, "first: 9 zeroed slots, one allocation (n %u)", n);
+    d[3].attached = 1; strcpy(d[3].name, "Zuse");
+    held = &d[3];                                       /* as g_zuse_dev, g_offered, g_spare hold one */
+
+    n = sk_v4_slots_get(&slots, &count, 9u, sizeof(drive));
+    CHECK(slots == (void *)d && n == 9u && allocs == 1u, "again, the same size: the same array, no allocation");
+
+    n = sk_v4_slots_get(&slots, &count, 17u, sizeof(drive));
+    CHECK(slots == (void *)d && allocs == 1u, "#178: asked for more: the same array, not replaced (allocations %u)", allocs);
+    CHECK(n == 9u, "#178: and only the 9 slots it has may be used, not 17 (n %u)", n);
+    CHECK(held->attached == 1 && strcmp(held->name, "Zuse") == 0 && held == &((drive *)slots)[3],
+          "#178: a pointer held into a slot is still the slot, with the drive in it");
+
+    {
+        void *none = 0; uint32_t c = 0;
+        unsigned before = allocs;
+        CHECK(sk_v4_slots_get(&none, &c, 0u, sizeof(drive)) == 0u && none == 0 && allocs == before,
+              "0 slots wanted: none to use, nothing allocated");
+    }
+    return DONE();
+}