Phase E: MINT -- real keypair, Zuse-signed DER cert, working default identity

capsule_mint_identity() (new capsule_mint.h/.c): mints a fresh identity
onto a blank/unminted thumbdrive -- real Ed25519 keypair from virtio_rng,
a fresh drive_uuid (independent random draw, not derived from the
identity seed, per FABRIC-3.md §F.8 decision 3), a Zuse-signed DER cert
in the CERTVERIFY format, and a small working default personality (a
real WELCOME word, not a stub -- FABRIC-3.md §F.6/§F.8's own "default
personality content" question stays open, but whatever mints today must
actually do something when RUNCAP births it). Refuses to overwrite a
drive that already reads as a recognized home-blocks drive, mirroring
WRITE(10)'s own refuse-on-non-blank posture (decided now, not just
"reasonable by analogy" as §F.8 left it).

x509_build_user_cert() (x509_ed25519.h/.c): the encode-side counterpart
to the existing decode functions (x509_extract_ed25519_pubkey(),
x509_verify_signature(), x509_extract_serial()) -- a minimal DER TLV
writer producing exactly the fields those functions read. Host-tested
round-trip against the real decoder before trusting it in the kernel,
including a high-bit-serial case that exercises the DER integer-padding
rule; all assertions pass (pubkey/serial round-trip, signature verifies
against the real issuer, correctly rejects the wrong key and a
corrupted signature).

New user_identity_seed_t (user_identity_seed.h): the on-disk record for
a minted identity's own keypair, same magic+version+fields+pad-to-4096+
real-CRC convention as zuse_cert_devblock_t and homeblocks_sig_t. Fixed
devblock layout: sig(1), cert(2), seed record(3), default personality(4).

New MINT word (mama_forth_words.c) and a small accessor
(sk_repl_get_attached_blk_dev(), repl.h/.c) exposing the currently
attached USB device regardless of home-blocks recognition -- MINT's own
target is a blank drive, which by definition never sets Phase D's
sk_repl_get_homeblocks_dev().

Verified end-to-end live in QEMU: MINT on a genuinely blank test drive,
then (after a detach/reattach so the sig cache picks up the fresh
header -- a known workflow gap, not fixed here, flagged for whoever
builds the real Console onboarding flow) RUNCAP birthed a VM from that
drive's own newly-minted content, and VM-EXECing its WELCOME word
printed the default personality banner. The full mint-to-birth Tripod
identity flow works end to end for the first time. Clean 3-architecture
regression boot confirms no side effects on normal boot.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019ZGkimpfyh63EZyRkNbkPD

include/starkernel/user_identity_seed.h

@@ -0,0 +1,49 @@
+/*
+ * user_identity_seed.h -- on-disk record format for a minted user
+ * identity's own keypair (FABRIC-3.md §F.8, decision 2), stored in the
+ * first devblock of a home-blocks drive's identity_src region
+ * (homeblocks_sig_t.identity_src_offset). The devblocks that follow it
+ * (identity_src_offset+1 .. identity_src_offset+identity_src_devblocks-1)
+ * hold this identity's own raw FORTH personality/init source, read by
+ * RUNCAP (capsule_runcap.h) at birth.
+ *
+ * Same raw-devblock, magic+version+fields+pad-to-4096, real-CRC-from-
+ * day-one convention as zuse_cert_devblock_t (zuse_cert_devblock.h) and
+ * homeblocks_sig_t -- a new, dedicated type rather than reusing
+ * zuse_cert_devblock_t, per this project's "give real-shaped data its
+ * own header" convention (Zuse's own record has no analogous public
+ * cert field; a regular user's does, stored separately in the cert
+ * region MINT also writes -- see homeblocks_sig_t.cert_offset).
+ */
+#ifndef STARKERNEL_USER_IDENTITY_SEED_H
+#define STARKERNEL_USER_IDENTITY_SEED_H
+
+#include <stdint.h>
+
+#define USER_IDENTITY_SEED_MAGIC \
+    ((uint32_t)'U' | ((uint32_t)'I' << 8) | ((uint32_t)'D' << 16) | ((uint32_t)'S' << 24))
+
+#define USER_IDENTITY_SEED_VERSION 1u
+
+typedef struct {
+    uint32_t magic;      /* USER_IDENTITY_SEED_MAGIC; anything else means
+                           * "not yet minted" (blank/foreign bytes), not a
+                           * format-corruption error. */
+    uint32_t version;    /* USER_IDENTITY_SEED_VERSION */
+    uint8_t  seed[32];   /* Ed25519 seed -- this identity's own private key.
+                           * Regular users are thumbdrive-resident (unlike
+                           * Zuse's system-resident one) -- this is the
+                           * only copy. */
+    uint8_t  pubkey[32]; /* Ed25519 public key derived from seed at mint
+                           * time -- same value the cert region's
+                           * SubjectPublicKeyInfo holds. */
+    uint64_t crc;        /* CRC-64/ISO (block_subsystem.h's compute_crc64())
+                           * over every byte of this struct up to (not
+                           * including) this field. */
+    uint8_t  _pad[4096 - (4 + 4 + 32 + 32 + 8)];
+} user_identity_seed_t;
+
+typedef char user_identity_seed_size_check[
+    (sizeof(user_identity_seed_t) == 4096) ? 1 : -1];
+
+#endif /* STARKERNEL_USER_IDENTITY_SEED_H */