fix(v4): the build refuses an Artemis cap its 32-bit block numbers cannot hold (#182)

sk_v4_grow.c's dev_rw4k computes 1 KiB block numbers as (uint32_t)devblock * 4,
which wraps past 4 TiB; Artemis is capped at 256 GiB (STICK_ARTEMIS_CAP_GIB,
kernel/Makefile), so it is latent -- but nothing said so where the cast is. Now
a compile-time check there (the C99 negative-array pattern the tree uses): a
cap past 4 TiB stops the build, and a cap not defined at all is an #error, not
a skipped check. gpt.c's casts are bounded by the disk's own size, itself a
32-bit count of 1 KiB blocks, checked before each: each now says so.

kernel/tests' check_cap, seen to fail first (a 4096 GiB cap built); now 3
checks, 0 failures -- 256 GiB builds, 4096 GiB and none refused. Built for
amd64, aarch64, riscv64. The plan's increments 2 and 3 marked.

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

kernel/src/v4/sk_v4_grow.c

@@ -26,6 +26,14 @@ static union {
 } g_hdr;
 static uint8_t g_move[4096];
 
+/* #182: the 1 KiB block numbers below are 32 bits ((uint32_t)devblock * 4u, as the block layer's
+ * are): enough for 4 TiB, and Artemis is capped at STICK_ARTEMIS_CAP_GIB (kernel/Makefile, 256 GiB).
+ * A cap past 4 TiB stops the build here instead of wrapping the block numbers. */
+#ifndef STICK_ARTEMIS_CAP_GIB
+#error "STICK_ARTEMIS_CAP_GIB (kernel/Makefile) must be defined: the check below depends on it"
+#endif
+typedef char sk_v4_grow_cap_fits_32_bit_blocks[(STICK_ARTEMIS_CAP_GIB * 1024ULL * 1024ULL <= 0xFFFFFFFFULL) ? 1 : -1];
+
 static int dev_rw4k(blkio_dev_t *dev, uint64_t devblock, uint8_t *buf, int write)
 {
     uint32_t base = (uint32_t)devblock * 4u, k;