Reorganize source tree: kernel/, v3/, v4/ split and board infrastructure

Source tree reorganization:
- Move StarForth v3 engine to v3/ (src/, include/, Makefile)
- Move kernel to kernel/ (src/, include/, linker/, Makefile)
- Create v4/ skeleton for F18-ISA golden model (DECOMPOSITION.md, JUSTIFICATION.md)
- Move FABRIC-0..4.md to docs/fabric/
- Move ONTOLOGY.md and ROADMAP.md to docs/

Board infrastructure:
- Add boards/ser5/, boards/raspi/, boards/milkv/, boards/zynq7020/
- Each board has board.mk (ISA, CPU flags, boot recipe) and README.md
- Root Makefile becomes thin dispatcher: boot_image, all, clean, docs take TARGET
- make boot_image TARGET=SER5|RASPI|MILKV builds one GPT/MBR image per board
- ZYNQ7020 target exists but stops with clear error (ARMv7 port not built yet)
- scripts/mkdiskimage.sh builds disk images for all boards

Docs pipeline:
- docs/book/ with LaTeX master (main.tex) and Makefile
- pandoc converts Markdown to LaTeX at build time
- Two Lua filters: table-widths.lua (wide tables wrap), code-breaks.lua (inline code breaks)
- make docs builds single PDF (754 pages, 0 missing characters)
- make docs TARGET=<board> adds board appendix
- build/docs/<book|board>/meta.tex stamps git commit into PDF

Bug fixes:
- 42 include paths that only worked by accident now use correct relative paths
- clang-18 hardcode replaced with configurable CC variable (fixed aarch64 build)
- Pi 5: kernel_2712.img linked at 0x80000, .bss zeroed, memory reserved
- Doxyfile, .clang-tidy, README.md, Kconfig paths updated

Verified:
- Hosted v3 build passes 1012 tests, 0 failures
- SER5 image boots in QEMU (OVMF), POST passes, K exact (65536 = Q48_ONE)
- Milk-V image boots in QEMU (OpenSBI + U-Boot + bootefi), POST passes
- make clean TARGET=<board> removes only that board and its ISA objects
- make all builds all boards, hosted v3, and docs in one run

Co-authored-by: Junie <junie@jetbrains.com>

kernel/src/homeblocks_sig.c

@@ -0,0 +1,87 @@
+/*
+  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.c - Drive signature check (FABRIC-2.md, Milestone 4).
+ * See starkernel/homeblocks_sig.h for the format and interface design.
+ */
+
+#include "starkernel/homeblocks_sig.h"
+
+#include <stddef.h>
+#include <string.h>
+
+#include "blkio.h"
+#include "block_subsystem.h" /* compute_crc64() -- same CRC-64/ISO this file's
+                              * check reuses rather than duplicating */
+
+uint64_t homeblocks_sig_compute_crc(const homeblocks_sig_t *sig) {
+    /* Covers every field up to but not including hdr_crc itself (and never
+     * _pad, which sits after it) -- offsetof is the exact boundary, not a
+     * hand-counted byte offset that could drift out of sync with the
+     * struct's own field list. */
+    size_t crc_span = offsetof(homeblocks_sig_t, hdr_crc);
+    return compute_crc64((const uint8_t *)sig, crc_span);
+}
+
+homeblocks_sig_result_t homeblocks_sig_check(struct blkio_dev *dev,
+                                              uint32_t sig_start_fblock,
+                                              homeblocks_sig_t *out_sig) {
+    uint8_t          buf[4096];
+    uint32_t         i;
+    homeblocks_sig_t local;
+    uint64_t         expected_crc;
+
+    if (!dev) return HOMEBLOCKS_SIG_READ_ERROR;
+
+    /* homeblocks_sig_t is exactly one 4KiB devblock; blkio's own unit is a
+     * 1KiB "forth block" (BLKIO_FORTH_BLOCK_SIZE), so the header spans 4
+     * consecutive reads starting at sig_start_fblock. */
+    for (i = 0; i < 4; i++) {
+        if (blkio_read((blkio_dev_t *)dev, sig_start_fblock + i,
+                        buf + (size_t)i * BLKIO_FORTH_BLOCK_SIZE) != BLKIO_OK) {
+            return HOMEBLOCKS_SIG_READ_ERROR;
+        }
+    }
+
+    /* Copy into a properly-aligned local rather than reinterpreting buf's
+     * address directly -- buf is only byte-aligned, and homeblocks_sig_t
+     * has uint64_t members; a raw cast would be a strict-aliasing and
+     * alignment violation for no benefit over one memcpy. */
+    memcpy(&local, buf, sizeof(local));
+
+    if (HOMEBLOCKS_SIG_GET_MAGIC(local.magic) != (uint32_t)(HOMEBLOCKS_SIG_MAGIC & 0xFFFFFFFFULL)) {
+        return HOMEBLOCKS_SIG_BLANK;
+    }
+
+    if (HOMEBLOCKS_SIG_GET_VERSION(local.magic) != HOMEBLOCKS_SIG_VERSION_0) {
+        return HOMEBLOCKS_SIG_BAD_VERSION;
+    }
+
+    expected_crc = homeblocks_sig_compute_crc(&local);
+    if (expected_crc != local.hdr_crc) {
+        return HOMEBLOCKS_SIG_BAD_CRC;
+    }
+
+    if (out_sig) *out_sig = local;
+    return HOMEBLOCKS_SIG_OK;
+}