wip(kernel): Artemis grows to fill the stick at its first boot -- metadata only, capped at 256 GiB (#92, Task 2) [skip ci]

Not yet accepted (plan 2026-10-29-stick-5, Task 2, step 3).
- gpt.c: the writer, gpt_grow_last() -- the backup entry array and header to
  the disk's end, the last partition to the last usable sector (whole MiB,
  capped), both headers' CRCs, the backup before the primary, the
  protective MBR's size; gpt_alternate_lba().
- sk_v4_grow.c: the volume grown to its partition -- its fence (the Artemis
  signature, Zuse's genesis marker) moved to the new top, the header last;
  the partition grown no further than the BAM tracks (the block subsystem
  places the fence by the device's size, so a volume must fill it).
- sk_v4_identity.c: a stick of 8 GB or more whose GPT ends short of the disk
  is grown before its partitions are opened; "Artemis: grown to fill the
  stick" / "under 8 GB -- not grown" / "past 256 GiB ... left unused".
- block_subsystem: blk_volume_compute_totals() (compute_totals_from_B,
  public) and blk_subsys_set_format_bam_for(); v3's behaviour unchanged.
- mkartemis --bam-for, mkdiskimage.sh --bam-for, kernel/Makefile
  STICK_GROW_MIN_BYTES / STICK_ARTEMIS_CAP_GIB: the stick's and data stick's
  Artemis BAM sized for 256 GiB (6144 devblocks). STICK.md 5a: 256 GiB (R1).

After: stick_grow PASS (16 GiB: 12129370 user blocks, 8 s POST to prompt;
second boot not grown, recognised; sfdisk --verify: no errors), stick_small
PASS, check_images.sh PASS.

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

v3/include/block_subsystem.h

@@ -591,6 +591,18 @@ int blk_subsys_add_raw_device(uint8_t *buf, uint32_t nblocks);
  * by the disk's owner before any write to that slot will succeed. */
 int blk_subsys_confirm_format(uint32_t lbn);
 
+/* blk_volume_compute_totals - a volume's tracked/total block counts from its
+ * total_devblocks, bam_devblocks, reloc_devblocks and meta_fence_blocks, as a
+ * fresh format computes them. For a volume grown to fill a larger partition
+ * (the v4 stick, plan 2026-10-29-stick-5, Task 2). */
+void blk_volume_compute_totals(blk_volume_meta_t *m);
+
+/* blk_subsys_set_format_bam_for - size the BAM of the next fresh format for a
+ * volume of `total_devblocks` 4 KiB devblocks, when that is more than the
+ * disk's own: the volume can later grow that far with no BAM of its own to
+ * move (mkartemis --bam-for). 0, the default, sizes it for the disk. */
+void blk_subsys_set_format_bam_for(uint64_t total_devblocks);
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif