Build DOE-PERSIST-TRIAL: block-backed DoE campaign persistence, live-verified

FABRIC-3.md §XXXV.3/.4 designed this and it was never built -- two live
campaigns got launched relying entirely on a live serial-log tail before
this was caught. doe_region.c/.h mirrors log_region.c's own growable-ring
pattern (own devblock_from_top fence at 98, past log_region's ceiling at
97) to persist one trial-summary record per completed trial straight to
disk/artemis.img, surviving past QEMU exit with no host required.

Verified end to end: booted amd64, called DOE-PERSIST-TRIAL at the
console, clean BYE, then read the raw disk bytes back with QEMU fully
dead -- confirmed the exact values passed in. Wired into both
multiuser-doe.4th and per-isa-doe.4th's trial markers. Clean build, zero
new warnings, all 3 ISAs; mkcapsule --lint passes both edited capsules.

FABRIC-3.md §XXXV.10 documents the fix and the root-cause correction: two
campaigns were launched on an unbuilt persistence design before this was
caught and fixed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWpNjdwPtFLuVLaAq44L9K

Makefile.starkernel

@@ -458,7 +458,8 @@ LOADER_SRCS_BASE := \
     $(KERNEL_SRC)/heartbeat.c \
     $(KERNEL_SRC)/homeblocks_sig.c \
     $(KERNEL_SRC)/artemis_sig.c \
-    $(KERNEL_SRC)/log_region.c
+    $(KERNEL_SRC)/log_region.c \
+    $(KERNEL_SRC)/doe_region.c
 
 LOADER_ASM := \
     $(KERNEL_SRC)/arch/$(ARCH)/boot.S \
@@ -516,7 +517,8 @@ KERNEL_SRCS_BASE := \
     $(KERNEL_SRC)/heartbeat.c \
     $(KERNEL_SRC)/homeblocks_sig.c \
     $(KERNEL_SRC)/artemis_sig.c \
-    $(KERNEL_SRC)/log_region.c
+    $(KERNEL_SRC)/log_region.c \
+    $(KERNEL_SRC)/doe_region.c
 
 KERNEL_ASM := $(wildcard $(KERNEL_SRC)/arch/$(ARCH)/*.S)