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

docs/v4.0.0/plans/2026-11-12-kernel-host-tests.md

@@ -32,7 +32,9 @@ fails half-way, a stop that spins) is still tested first and seen to fail. v4 ha
 - [x] 2. The xHCI configuration-descriptor walk moved out of the event loop (`usb_config.c`,
       `test_usb_config`), then #170 and #171 tested first (10 failures against the old decision)
       and fixed.
-- [ ] 3. #176 (seeds wiped on every exit) -- the approach to rule. [x] #177 (the fatal stops halt:
+- [x] 3. #176 (seeds wiped on every exit; option 1, Captain Bob 2026-11-12: "1 is great" -- the
+      caller owns the buffer: `test_sk_v4_mint`, 7 failures before; `test_sk_v4_identity`, through the
+      real drive-arrival path, 5 failures before). [x] #177 (the fatal stops halt:
       `sk_v4_stop.c`, `test_sk_v4_stop`, a spin seen failing). [x] #178 (the slot array:
       `sk_v4_slots.c`, `test_sk_v4_slots`, 4 failures against the old allocation). [x] #182 (the
       cap checked at compile time: `check_cap`, a 4 TiB cap seen building before).