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

Makefile.starkernel

@@ -456,7 +456,8 @@ LOADER_SRCS_BASE := \
     $(KERNEL_SRC)/repl.c \
     $(KERNEL_SRC)/doe_log.c \
     $(KERNEL_SRC)/heartbeat.c \
-    $(KERNEL_SRC)/homeblocks_sig.c
+    $(KERNEL_SRC)/homeblocks_sig.c \
+    $(KERNEL_SRC)/artemis_sig.c
 
 LOADER_ASM := \
     $(KERNEL_SRC)/arch/$(ARCH)/boot.S \
@@ -511,7 +512,8 @@ KERNEL_SRCS_BASE := \
     $(KERNEL_SRC)/repl.c \
     $(KERNEL_SRC)/doe_log.c \
     $(KERNEL_SRC)/heartbeat.c \
-    $(KERNEL_SRC)/homeblocks_sig.c
+    $(KERNEL_SRC)/homeblocks_sig.c \
+    $(KERNEL_SRC)/artemis_sig.c
 
 KERNEL_ASM := $(wildcard $(KERNEL_SRC)/arch/$(ARCH)/*.S)