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

src/starkernel/repl.c

@@ -88,6 +88,18 @@ const homeblocks_sig_t *sk_repl_get_homeblocks_sig(void) {
     return g_homeblocks_sig_valid ? &g_homeblocks_sig : (void *)0;
 }
 
+/* The currently attached USB block device, regardless of whether it
+ * checks out as a recognized home-blocks drive -- MINT (§F.8/§F.19)
+ * targets a blank/unminted drive, which by definition never sets
+ * g_homeblocks_dev above (that only latches on HOMEBLOCKS_SIG_OK).
+ * Set once blk_subsys_attach_device() succeeds below, cleared on detach
+ * alongside g_homeblocks_dev. */
+static blkio_dev_t *g_attached_blk_dev = (void *)0;
+
+blkio_dev_t *sk_repl_get_attached_blk_dev(void) {
+    return g_attached_blk_dev;
+}
+
 /*===========================================================================
  * Idle heartbeat service
  *
@@ -174,6 +186,7 @@ static void sk_repl_idle(VM *active_vm)
         }
         if (rc == 0 && blk_subsys_attach_device(&usb_blk_dev) == BLK_OK) {
             xdev->bot_msc_attached = 1;
+            g_attached_blk_dev     = &usb_blk_dev;
         } else {
             console_println("xhci: USB MSC block-subsystem attach failed");
         }
@@ -193,6 +206,9 @@ static void sk_repl_idle(VM *active_vm)
             g_homeblocks_dev       = (void *)0;
             g_homeblocks_sig_valid = 0;
         }
+        if (g_attached_blk_dev == &usb_blk_dev) {
+            g_attached_blk_dev = (void *)0;
+        }
     }
 
     /* FABRIC.md/FABRIC-2.md Section V item 6: "a cheap 'anything dirty?