wip(kernel): the proxy Zuse -- a stick's first boot mints its own Zuse (#92, Task 1) [skip ci]

Work in progress on StarForth-v4.0.0; not yet accepted (plan
2026-10-29-stick-5, Task 1).

- zuse_genesis_marker.h: the marker's version 2 -- the proxy's key and the
  real Zuse's -- beside version 1.
- capsule_zuse_boot.c: capsule_zuse_boot_check_key() (the identity-record
  half of the check) and capsule_zuse_boot_genesis_mint() (the mint half of
  v3's genesis) split out; v3's two callers unchanged in behaviour.
- sk_v4_marker.c: the v4 boot reads version 1 as a proxy with no real Zuse,
  and writes version 2.
- sk_v4_identity.c: no marker and a blank identity partition -> genesis:
  mint, read back, then the marker; the proxy logs in. A marker with a real
  key: no session for the proxy (the fixed screen is Task 5).
- qemu_session.py: boot()/shutdown(), so a mode can boot twice; modes
  genesis and stick_genesis; stick_boot now expects genesis and the prompt.

Logs: genesis on amd64 before the change (both boots: "no genesis on this
disk", FAIL) and after (minted, then recognised: PASS). The DoE CSV of
032532 is from a run whose QEMU never started (no display; its empty log
directory was removed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

kernel/include/starkernel/v4/sk_v4_marker.h

@@ -0,0 +1,30 @@
+/* sk_v4_marker.h -- the genesis marker, as the v4 boot reads and writes it:
+ * version 2, the stick's proxy Zuse and the real Zuse (docs/v4.0.0/STICK.md 6,
+ * 6a; plan 2026-10-29-stick-5, Task 1). On Artemis's metadata fence,
+ * devblock 0 from the top (blk_meta_zone_read/write), as v3's version 1.
+ *
+ * Built only with STARFORTH_V4=1 (Kconfig.kernel).
+ */
+#ifndef STARKERNEL_V4_SK_V4_MARKER_H
+#define STARKERNEL_V4_SK_V4_MARKER_H
+
+#include <stdint.h>
+
+typedef struct {
+    int     present;      /* a marker is on the fence: genesis has happened */
+    uint8_t proxy[32];    /* the stick's proxy Zuse */
+    int     has_real;     /* the real Zuse is minted, on a second stick */
+    uint8_t real[32];     /* her key, when has_real */
+} sk_v4_marker_t;
+
+/* The marker on the first disk slot's fence. A version 1 marker (v3's, the
+ * minted pair's) reads as a proxy with no real Zuse. Returns 0 with @p m
+ * filled in; -1 when there is none or it fails its checks (@p m then reads
+ * present = 0). */
+int sk_v4_marker_read(sk_v4_marker_t *m);
+
+/* Write @p m to the fence, always as version 2. Returns 0, or -1 on a write
+ * error. */
+int sk_v4_marker_write(const sk_v4_marker_t *m);
+
+#endif /* STARKERNEL_V4_SK_V4_MARKER_H */