Phase 8 C (5/n): Zuse's cert wired to the fence -- first-boot mint works

Replaces the crashed NVRAM approach entirely. New
include/starkernel/zuse_cert_devblock.h: a standalone on-disk record
(magic + version + 32-byte seed + 32-byte pubkey + a real CRC-64/ISO
from day one, same discipline homeblocks_sig_t established) occupying
devblock_from_top=0 of the fence. Its own header, not inlined at the
boot call site, since the still-open MINT word will be a second
consumer of this exact format.

kernel_main.c's mint-or-load logic now reads the fence, installs an
existing valid cert, or mints fresh via virtio_rng+ed25519_keygen and
writes it. Runs right after virtio_rng_init(), before
capsule_birth_mama() -- unlike the crashed NVRAM attempt, raw block I/O
against Artemis's already-proven device has no boot-timing risk, so the
earlier "re-invoke ACL-ZUSE-BOOT after Mama birth" workaround is gone;
ACL.4th's self-activating ACL-ZUSE-BOOT sees a correct cert on its one
ordinary pass.

Verified independently across every real scenario, never trusting the
kernel's own report: fresh mint decodes correctly on disk with a CRC
confirmed by a from-scratch Python re-implementation of the algorithm;
a reboot without reformatting loads back byte-for-byte identical
seed/pubkey (genuinely "mint once, ever"); a pre-fence volume refuses
cleanly (no crash, no silent data loss, honest "not persistent"
reporting); the real, untouched disk/artemis.img exercises the same
graceful-refusal path identically on all three architectures.

Phase 8's core arc is now functionally complete: real entropy -> real
signing -> real anti-file block-native persistence -> a first-boot mint
that survives reboots. Still open: the ongoing MINT word for minting
additional regular users. Documented in FABRIC-3.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd

include/starkernel/zuse_cert_devblock.h

@@ -0,0 +1,46 @@
+/*
+ * zuse_cert_devblock.h -- on-disk record format for Zuse's cert, stored
+ * in devblock_from_top=0 of the top-of-device system-metadata fence
+ * (block_subsystem.h's blk_meta_zone_read()/write(), Phase 8, FABRIC-3.md
+ * §C). Raw, unpacked 4 KiB devblock -- same convention as the volume
+ * header itself (magic + version + fields + pad-to-4096, real CRC from
+ * day one, matching homeblocks_sig_t's own precedent for exactly this
+ * reason: this gates a real security check, not a placeholder).
+ *
+ * Deliberately its own header, not inlined at the one call site that
+ * uses it today (kernel_main.c's first-boot mint-or-load): the ongoing
+ * `MINT` word (still open, FABRIC-3.md) will be a second consumer of
+ * this exact format later, and the format should be stable and
+ * documented once rather than ad-hoc.
+ */
+#ifndef STARKERNEL_ZUSE_CERT_DEVBLOCK_H
+#define STARKERNEL_ZUSE_CERT_DEVBLOCK_H
+
+#include <stdint.h>
+
+/* Packed via shifts, not a hand-computed hex literal -- this project's
+ * own standing lesson about hand-derived numeric constants in this
+ * class of code (see FABRIC-3.md's Ed25519/scalar25519 writeups). */
+#define ZUSE_CERT_DEVBLOCK_MAGIC \
+    ((uint32_t)'Z' | ((uint32_t)'U' << 8) | ((uint32_t)'S' << 16) | ((uint32_t)'E' << 24))
+
+#define ZUSE_CERT_DEVBLOCK_VERSION 1u
+
+typedef struct {
+    uint32_t magic;      /* ZUSE_CERT_DEVBLOCK_MAGIC; anything else means
+                           * "not a real cert yet" (blank/foreign bytes),
+                           * not a format-corruption error */
+    uint32_t version;    /* ZUSE_CERT_DEVBLOCK_VERSION */
+    uint8_t  seed[32];   /* Ed25519 seed -- the private identity */
+    uint8_t  pubkey[32]; /* Ed25519 public key derived from seed at mint time */
+    uint64_t crc;        /* CRC-64/ISO (block_subsystem.h's compute_crc64())
+                           * over every byte of this struct up to (not
+                           * including) this field -- real from day one,
+                           * this gates a real security check */
+    uint8_t  _pad[4096 - (4 + 4 + 32 + 32 + 8)];
+} zuse_cert_devblock_t;
+
+_Static_assert(sizeof(zuse_cert_devblock_t) == 4096,
+               "zuse_cert_devblock_t must be exactly one 4 KiB devblock");
+
+#endif /* STARKERNEL_ZUSE_CERT_DEVBLOCK_H */