Artemis genesis stamp: fix a BAM-corrupting offset before it ever ran (FABRIC-3.md §XXVI follow-on, Step 3)

Step 3: one-time artemis_sig_t genesis stamp, written once
kernel_main.c's virtio-blk path confirms Artemis's own disk, so the disk
image is later recognizable generically (repl.c's idle-loop USB-MSC scan,
built in the prior commit) regardless of which bus found it.

Correction made before this ever touched the real disk: the signature's
first design (committed in 29b6789) placed it at a fixed bottom-of-device
forth-block (4, devblock 1) -- copying homeblocks_sig_t's own convention,
which is safe for a raw identity thumbdrive but not for Artemis's own
disk. Artemis's disk is block_subsystem.c's own STFR/v2-formatted volume:
devblock 0 holds that format's header and devblock 1 is the FIRST
DEVBLOCK OF THE LIVE BAM (blk_compute_fresh_geometry(): bam_start = 1).
The original design would have overwritten Artemis's live allocation map
on the very first real boot. Caught via direct cross-reference against
block_subsystem.c before the genesis-stamp call site was ever run against
the real image -- no corruption occurred.

Fixed by moving the header to a fixed offset from the END of the device
instead (ARTEMIS_SIG_DEVBLOCK_FROM_TOP=64), the same top-of-device region
block_subsystem.c's own meta_fence_blocks reservation (128 devblocks)
already carves out for system metadata, and where Zuse's genesis marker/
eligibility list already live -- but computed independently via
blkio_info() rather than through blk_meta_zone_*(), since that accessor
needs an already-attached, format-detected slot, which is exactly the
state pre-attach generic discovery doesn't have yet. Picked well clear of
Zuse's two tenants (devblock_from_top 0 and 1+, open-ended) so the two
subsystems' independent math can never collide.

Also reordered kernel_main.c: rng_init() now runs before the Artemis
virtio-blk block (was after) -- the genesis stamp needs rng_get_bytes()
for disk_uuid, and the original order would have failed the stamp on
every boot.

Verified live: booted amd64 against the real disk/artemis.img twice --
first boot logs "Artemis: genesis signature stamped" (confirmed blank at
the target offset beforehand via a host-side read), second boot on the
now-stamped image logs no re-stamp (idempotent, CRC/read-back verified)
-- both boots and aarch64/riscv64 (against the same now-stamped image)
all still report "Artemis: 22998 data blocks" / "PASS: persist-read"
unchanged, confirming the BAM and data pool were never touched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

src/starkernel/artemis_sig.c

@@ -34,6 +34,9 @@
 #include "block_subsystem.h" /* compute_crc64() -- same CRC-64/ISO
                               * homeblocks_sig.c's own check reuses,
                               * not duplicated here either */
+#include "starkernel/rng.h"  /* rng_get_bytes() -- same entropy source
+                              * capsule_mint.c already uses for an
+                              * identity thumbdrive's own drive_uuid */
 
 uint64_t artemis_sig_compute_crc(const artemis_sig_t *sig) {
     /* Covers every field up to but not including hdr_crc itself (and never
@@ -43,15 +46,42 @@ uint64_t artemis_sig_compute_crc(const artemis_sig_t *sig) {
     return compute_crc64((const uint8_t *)sig, crc_span);
 }
 
+/* Resolve ARTEMIS_SIG_DEVBLOCK_FROM_TOP (a distance from the last physical
+ * devblock) to an absolute starting forth-block on `dev`, using only
+ * blkio_info() -- no attach or format-detection required, matching
+ * block_subsystem.c's own devblock_from_top arithmetic
+ * (total_devblocks - 1 - devblock_from_top) exactly, but computed
+ * independently since blk_meta_zone_*() isn't reachable pre-attach. The
+ * floor division (total forth-blocks / 4) matches block_subsystem.c's own
+ * udiv_floor(total_blkio_blocks_1k, 4) -- plain unsigned integer division
+ * already floors, so no separate helper is needed here.
+ *
+ * @return 0 and *out_fblock set on success; -1 if the device is too small
+ *         to hold the reserved offset at all (couldn't happen on a real
+ *         Artemis disk, but a foreign/tiny device must never underflow
+ *         into a wild block). */
+static int artemis_sig_start_fblock(struct blkio_dev *dev, uint32_t *out_fblock) {
+    blkio_info_t info;
+    if (blkio_info((blkio_dev_t *)dev, &info) != BLKIO_OK) return -1;
+
+    uint32_t total_devblocks = info.total_blocks / 4u;
+    if (total_devblocks < ARTEMIS_SIG_DEVBLOCK_FROM_TOP + 1u) return -1;
+
+    uint32_t devblock_idx = total_devblocks - 1u - ARTEMIS_SIG_DEVBLOCK_FROM_TOP;
+    *out_fblock = devblock_idx * 4u;
+    return 0;
+}
+
 artemis_sig_result_t artemis_sig_check(struct blkio_dev *dev,
-                                        uint32_t sig_start_fblock,
                                         artemis_sig_t *out_sig) {
     uint8_t          buf[4096];
     uint32_t         i;
+    uint32_t         sig_start_fblock;
     artemis_sig_t    local;
     uint64_t         expected_crc;
 
     if (!dev) return ARTEMIS_SIG_READ_ERROR;
+    if (artemis_sig_start_fblock(dev, &sig_start_fblock) != 0) return ARTEMIS_SIG_READ_ERROR;
 
     /* artemis_sig_t is exactly one 4KiB devblock; blkio's own unit is a
      * 1KiB "forth block" (BLKIO_FORTH_BLOCK_SIZE), so the header spans 4
@@ -85,3 +115,47 @@ artemis_sig_result_t artemis_sig_check(struct blkio_dev *dev,
     if (out_sig) *out_sig = local;
     return ARTEMIS_SIG_OK;
 }
+
+int artemis_sig_genesis_stamp(struct blkio_dev *dev) {
+    if (!dev) return -1;
+
+    uint32_t sig_start_fblock;
+    if (artemis_sig_start_fblock(dev, &sig_start_fblock) != 0) return -1;
+
+    artemis_sig_t sig;
+    memset(&sig, 0, sizeof(sig));
+    sig.magic = ARTEMIS_SIG_PACK(ARTEMIS_SIG_VERSION_0);
+    if (rng_get_bytes(sig.disk_uuid, sizeof(sig.disk_uuid)) != 0) return -1;
+    sig.genesis_time_ns    = 0; /* no monotonic-ns source yet, same open
+                                 * item as homeblocks_sig_t's own
+                                 * minted_time_ns field */
+    sig.metadata_devblocks = 1; /* just this header devblock for now --
+                                 * grows to cover the log region once
+                                 * step 4 allocates it (a normal rewrite
+                                 * of this same one-header location) */
+    sig.log_region_offset    = 0; /* not yet allocated */
+    sig.log_region_devblocks = 0;
+    sig.hdr_crc = artemis_sig_compute_crc(&sig);
+
+    uint8_t buf[4096];
+    memcpy(buf, &sig, sizeof(sig));
+
+    uint32_t i;
+    for (i = 0; i < 4; i++) {
+        if (blkio_write((blkio_dev_t *)dev, sig_start_fblock + i,
+                         buf + (size_t)i * BLKIO_FORTH_BLOCK_SIZE) != BLKIO_OK) {
+            return -1;
+        }
+    }
+    (void)blkio_flush((blkio_dev_t *)dev);
+
+    /* Read-back verification, same discipline as capsule_mint.c's own
+     * verify_mint() -- a successful blkio_write() return only means the
+     * command chain completed, not that the bytes survive to be read
+     * back correctly. */
+    artemis_sig_t verify;
+    if (artemis_sig_check(dev, &verify) != ARTEMIS_SIG_OK) return -1;
+    if (memcmp(&verify, &sig, sizeof(sig)) != 0) return -1;
+
+    return 0;
+}