Add homeblocks_sig.h: drive signature header struct (Phase 8 kickoff)

Implements the design from the previous commit as-is: homeblocks_sig_t
(4096 bytes, magic+version+drive_uuid+timestamp+cert/blockmap offset
reservations+real hdr_crc), HOMEBLOCKS_SIG_PACK/_GET_MAGIC/_GET_VERSION
macros mirroring CAPSULE_MAGIC_PACK's bit layout, and a C99
compile-time size assertion matching stadium.h's own discipline.

Verified standalone (sizeof == 4096, clean under -std=c99 -Wall
-Wextra -Werror) -- nothing consumes this header yet, so no
functional kernel change and no 3-arch acceptance boot needed for this
step.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CXjAPTEKrgY2Mrk25KoLDn

FABRIC-3.md

@@ -203,7 +203,14 @@ decisions get added here, not to `FABRIC-2.md`. Follow the same discipline `FABR
   generated yet (Milestone 6), so this reserves the *shape* of where a cert will attach
   without committing to a cert format that doesn't exist. Same reasoning for
   `blockmap_offset`/`blockmap_devblocks` against Milestone 3's still-open block-map design.
-  Not yet written to a header file — presented for confirmation before implementation.
+  **Implemented (2026-08-26):** `include/starkernel/homeblocks_sig.h` — `homeblocks_sig_t` +
+  `HOMEBLOCKS_SIG_PACK`/`_GET_MAGIC`/`_GET_VERSION` macros, a C99 compile-time size assertion
+  (same discipline `stadium.h`'s own header-size checks use), and the identical field layout
+  shown above. Verified standalone: `sizeof(homeblocks_sig_t) == 4096`, compiles clean under
+  `-std=c99 -Wall -Wextra -Werror`. Not yet consumed by any code — nothing in the block
+  subsystem or xHCI driver reads or writes it yet, so no functional kernel change and no
+  3-arch acceptance boot needed for this step; that starts with the signature-check
+  implementation, the next punch-list item below.
 
 - [ ] Implement the signature check, called before any write path touches a newly-inserted
   drive.

include/starkernel/homeblocks_sig.h

@@ -0,0 +1,125 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+/**
+ * homeblocks_sig.h - Home-blocks drive signature format (FABRIC-3.md,
+ * Milestone 4, Phase 8 kickoff)
+ *
+ * Identifies and authenticates a physical thumb drive as a legitimate
+ * LithosAnanke home-blocks drive, before any write path touches it.
+ * Lives at the first 4KiB devblock of the drive's GPT metadata partition
+ * (the ~1GB partition decided 2026-08-22) -- this header's own format does
+ * not depend on the GPT parser that locates that partition; it is simply
+ * what gets written starting at that partition's first devblock, once
+ * something can find it.
+ *
+ * Mirrors two existing precedents exactly, not invented fresh:
+ *   - CAPSULE_MAGIC_PACK's bit-packed magic (starkernel/capsule.h)
+ *   - blk_volume_meta_t's magic+version+fields+pad-to-4096 structural
+ *     convention (block_subsystem.h)
+ *
+ * Deliberately narrow in scope: this header identifies/authenticates the
+ * DRIVE only. It does not invent the on-drive block-map format (Milestone
+ * 3, still undesigned) or the credential/cert format (Milestone 6, blocked
+ * on a real CA that does not exist yet) -- it only reserves offset/size
+ * pointers to where those will attach, so this format does not need to be
+ * revisited when those get designed.
+ */
+
+#ifndef STARKERNEL_HOMEBLOCKS_SIG_H
+#define STARKERNEL_HOMEBLOCKS_SIG_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*===========================================================================
+ * Magic Field Packing -- same bit layout convention as CAPSULE_MAGIC_PACK
+ *
+ * bits  0..31 : 'LAHB' (0x4248414C little-endian) -- LithosAnanke Home Blocks
+ * bits 32..39 : version (0 for v0)
+ * bits 40..63 : reserved (zero)
+ *===========================================================================*/
+
+#define HOMEBLOCKS_SIG_MAGIC     0x4248414CULL /* 'LAHB', same little-endian ASCII
+                                                 * packing as CAPSULE_DESC_MAGIC's 'CAPS' */
+#define HOMEBLOCKS_SIG_VERSION_0 0
+
+#define HOMEBLOCKS_SIG_PACK(ver) \
+    (HOMEBLOCKS_SIG_MAGIC | ((uint64_t)(ver) << 32))
+
+#define HOMEBLOCKS_SIG_GET_MAGIC(m)   ((uint32_t)((m) & 0xFFFFFFFFULL))
+#define HOMEBLOCKS_SIG_GET_VERSION(m) ((uint8_t)(((m) >> 32) & 0xFF))
+
+/*===========================================================================
+ * homeblocks_sig_t - drive signature header (exactly one 4KiB devblock)
+ *===========================================================================*/
+
+typedef struct {
+    uint64_t magic;              /* HOMEBLOCKS_SIG_PACK(...) */
+    uint8_t  drive_uuid[16];     /* Unique per-mint instance id -- Phase 8 mints multiple
+                                  * distinct drives, needs something to tell them apart. */
+    uint64_t minted_time_ns;     /* Monotonic timestamp at mint time. */
+    uint64_t metadata_devblocks; /* Size of this GPT metadata partition, in 4KiB devblocks --
+                                  * sanity/bounds check against the GPT entry once a parser
+                                  * exists. */
+
+    uint32_t cert_offset;        /* Devblock offset within this partition where the CA-signed
+                                  * cert blob starts; 0 = not yet minted. Format not decided --
+                                  * Milestone 6's real CA does not exist yet. */
+    uint32_t cert_devblocks;     /* Size reserved for the cert blob, in devblocks. */
+
+    uint32_t blockmap_offset;    /* Devblock offset where the block-map starts; 0 = not yet
+                                  * minted. Format not decided -- Milestone 3, still open. */
+    uint32_t blockmap_devblocks; /* Size reserved for the block-map, in devblocks. */
+
+    uint64_t hdr_crc;            /* REAL from day one, not a placeholder like
+                                  * blk_volume_meta_t's "unused yet" hdr_crc -- this header's
+                                  * whole job is gating a warn-and-refuse security check
+                                  * against blank/foreign/unrecognized media, so the crc has
+                                  * to actually work. Computed over every field above this
+                                  * one; callers must fill every other field before computing
+                                  * or verifying it. */
+
+    /* Padding to keep the header exactly one 4KiB devblock. */
+    uint8_t _pad[4096 - (
+                     8 +           /* magic */
+                     16 +          /* drive_uuid */
+                     8 +           /* minted_time_ns */
+                     8 +           /* metadata_devblocks */
+                     4 + 4 +       /* cert_offset, cert_devblocks */
+                     4 + 4 +       /* blockmap_offset, blockmap_devblocks */
+                     8             /* hdr_crc */
+                 )];
+} homeblocks_sig_t;
+
+/* C99-portable compile-time size assertion (no _Static_assert -- that's C11),
+ * same discipline stadium.h's own header-size checks already use. */
+typedef char homeblocks_sig_size_check[(sizeof(homeblocks_sig_t) == 4096) ? 1 : -1];
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* STARKERNEL_HOMEBLOCKS_SIG_H */