fix(v4): the identity code wipes Zuse's seed after every drive it handles (#176, the identity code)

sk_v4_genesis never wiped its seed or its read-back; sk_v4_drive_arrived
never wiped the proxy's seed after her login; sk_v4_real_zuse_arrived wiped
the seed only on a match. (The logged-in Zuse's own key stays in g_zuse_seed,
by design.)

Option 1: the three use one buffer the identity module owns, g_seeds (seed,
then its read-back), and sk_v4_drive_arrived -- which every drive goes through,
and which alone calls the other two -- runs the drive's handling (now
handle_drive()) and then wipes it, whichever way it went.
sk_v4_identity_seeds() lets the host tests read it.

kernel/tests/test_sk_v4_identity.c, 10 checks, through the real drive-arrival
path: a fake xHCI controller marks a slot attach-pending, the identity code's
idle beat handles it, the real gpt.c reads the stick's table from the RAM disk;
the block layer, signatures, key mint, marker and console faked. Genesis; a
genesis not read back; the proxy logging in; a real Zuse named, then her stick
logging in. Against the code before the wipe: 5 failures (every case's seeds
left). Now: 0. The harness gains fake_disk_bind. Built for amd64, aarch64,
riscv64. #176 done with 2bac2ac8 (the mint).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017RzZgrf7V7Zk2iczBXDRbm

kernel/include/starkernel/v4/sk_v4_identity.h

@@ -56,6 +56,10 @@ void sk_v4_identity_set_marker(const sk_v4_marker_t *m);
  * written its GPT and identity (it is then recognised as the real Zuse's). */
 void sk_v4_identity_recheck_offered(void);
 
+/* The 64 bytes the identity code mints and reads Zuse's seed into while it handles a drive (seed,
+ * then its read-back): zero whenever no drive is being handled (#176). For the host tests. */
+const uint8_t *sk_v4_identity_seeds(void);
+
 /* Headless: no prompt, and what is typed is read and dropped, until Zuse's
  * session begins (v3's sk_repl_headless_wait()). */
 void sk_v4_identity_wait(void);