Artemis bus-agnostic discovery: signature format + idle-loop generalization (FABRIC-3.md §XXVI follow-on)

Step 1: new artemis_sig_t header format (magic 'ARTM', sibling to
homeblocks_sig_t, distinct so a generic scan can tell Artemis's own disk
apart from an identity thumbdrive by content alone) -- artemis_sig.h/.c,
wired into Makefile.starkernel.

Step 2: sk_repl_idle()'s existing per-USB-MSC-slot attach handling (the
pattern WIREBIND already uses for identity thumbdrives) now also checks
for the ARTM signature whenever a device's home-blocks check comes back
BLANK. On a match, once Artemis's own storage-attach round-trip
(HERA-BLK-ATTACH-REQ/BLK-ATTACH-ACK) confirms success,
capsule_zuse_boot_load_root_pubkey() runs -- the same call kernel_main.c's
synchronous QEMU-only virtio-blk path already makes, now reachable
without a hardcoded PCI vendor/device scan. That function is already
idempotent (no-op once zuse_root_pubkey_known is set), so no boot
restructuring was needed despite the initial concern that deferring
Artemis discovery to the idle loop would require one.

Verified: clean build + QEMU boot to [zuse@Hera] ok> on all three
architectures, zero regression to the existing virtio-blk/Zuse-thumbdrive
attach path.

Steps 3 (genesis-stamping onto disk/artemis.img) and 4 (growable
production log-persistence region) not yet started.

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

src/starkernel/artemis_sig.c

@@ -0,0 +1,87 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+/**
+ * artemis_sig.c - Artemis disk signature check (FABRIC-3.md §XXVI follow-on).
+ * See starkernel/artemis_sig.h for the format and interface design.
+ */
+
+#include "starkernel/artemis_sig.h"
+
+#include <stddef.h>
+#include <string.h>
+
+#include "blkio.h"
+#include "block_subsystem.h" /* compute_crc64() -- same CRC-64/ISO
+                              * homeblocks_sig.c's own check reuses,
+                              * not duplicated here either */
+
+uint64_t artemis_sig_compute_crc(const artemis_sig_t *sig) {
+    /* Covers every field up to but not including hdr_crc itself (and never
+     * _pad, which sits after it) -- offsetof is the exact boundary, same
+     * discipline as homeblocks_sig_compute_crc(). */
+    size_t crc_span = offsetof(artemis_sig_t, hdr_crc);
+    return compute_crc64((const uint8_t *)sig, crc_span);
+}
+
+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;
+    artemis_sig_t    local;
+    uint64_t         expected_crc;
+
+    if (!dev) 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
+     * consecutive reads starting at sig_start_fblock -- same pattern as
+     * homeblocks_sig_check(). */
+    for (i = 0; i < 4; i++) {
+        if (blkio_read((blkio_dev_t *)dev, sig_start_fblock + i,
+                        buf + (size_t)i * BLKIO_FORTH_BLOCK_SIZE) != BLKIO_OK) {
+            return ARTEMIS_SIG_READ_ERROR;
+        }
+    }
+
+    /* Copy into a properly-aligned local rather than reinterpreting buf's
+     * address directly -- same strict-aliasing/alignment reasoning as
+     * homeblocks_sig_check(). */
+    memcpy(&local, buf, sizeof(local));
+
+    if (ARTEMIS_SIG_GET_MAGIC(local.magic) != (uint32_t)(ARTEMIS_SIG_MAGIC & 0xFFFFFFFFULL)) {
+        return ARTEMIS_SIG_BLANK;
+    }
+
+    if (ARTEMIS_SIG_GET_VERSION(local.magic) != ARTEMIS_SIG_VERSION_0) {
+        return ARTEMIS_SIG_BAD_VERSION;
+    }
+
+    expected_crc = artemis_sig_compute_crc(&local);
+    if (expected_crc != local.hdr_crc) {
+        return ARTEMIS_SIG_BAD_CRC;
+    }
+
+    if (out_sig) *out_sig = local;
+    return ARTEMIS_SIG_OK;
+}