fix: MINT-ZUSE says the stick was erased when it was (#168)

failed() printed "MINT-ZUSE: failed -- nothing changed" on every failure
after YES, though by then gpt_write_one() had rewritten the stick's partition
table and the identity partition's first blocks had been zeroed.

gpt_write_one() now tells the two apart: GPT_EIO when its first write failed
(nothing written), GPT_PART when a later write or flush failed (the disk is
changed). MINT-ZUSE keeps "failed -- nothing changed" for GPT_EIO only, and
says "failed -- the stick was erased; Zuse not minted" for every failure
after the table is written. The marker is still written last.

Tested first, seen to fail: qemu_session.py mint_part_fail (writes fail only
at the identity partition's first sector, LBA 2048) -- "nothing changed",
logs/20261112-122132. After: mint_part_fail PASS; mint_fail (every write
fails) still "nothing changed", PASS; replace (the success path) PASS. Their
logs here. FABRIC-3 XLVIII.20.

Captain Bob, 2026-11-12: "start #168".

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

kernel/include/starkernel/gpt.h

@@ -24,7 +24,8 @@ typedef struct {
 #define GPT_OK    0      /* a valid GPT */
 #define GPT_NONE  1      /* not a GPT disk -- no protective MBR -- (or one whose sectors are not 512 bytes): read it raw */
 #define GPT_BAD   2      /* a protective MBR, but its header or entry array does not check out: not used */
-#define GPT_EIO   3      /* its first block could not be read: not used */
+#define GPT_EIO   3      /* its first block could not be read: not used; gpt_write_one: nothing was written */
+#define GPT_PART  4      /* gpt_write_one only: a write failed after others were made -- the disk is changed (#168) */
 
 /* Read `disk`'s protective MBR, primary GPT header and partition-entry array,
  * checking both CRC-32s, the revision, the sizes and the geometry. On GPT_OK,
@@ -59,7 +60,10 @@ int gpt_grow_last(blkio_dev_t *disk, const uint8_t type[16], uint64_t max_part_s
  * entries, the backup header and entries at the disk's end, the protective
  * MBR written last (the old one zeroed first, so a write torn anywhere leaves
  * no table). Nothing inside the partition is written. GPT_OK; GPT_BAD for a
- * disk whose sectors are not 512 bytes or that is too small; GPT_EIO. */
+ * disk whose sectors are not 512 bytes or that is too small (nothing written);
+ * GPT_EIO when its first write failed (nothing written, the disk as it was);
+ * GPT_PART when a later write or a flush failed (the disk is changed: its old
+ * table is gone). The caller says which (#168). */
 int gpt_write_one(blkio_dev_t *disk, const uint8_t type[16], uint64_t part_sectors, uint8_t disk_guid[16]);
 
 /* The index of the first partition of `type`, or -1. */