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/include/starkernel/virtio_blk.h

@@ -0,0 +1,57 @@
+/*
+ * virtio_blk.h — Virtio 1.0 block device driver for StarKernel
+ *
+ * Presents a blkio_dev_t interface for attachment to the StarForth
+ * block subsystem via blk_subsys_attach_device().
+ *
+ * Only one virtio-blk device is supported (the Artemis disk).
+ * I/O is synchronous (polling) — no interrupts, no DMA descriptors
+ * beyond the split virtqueue shared memory.
+ *
+ * Artemis disk identification heuristic:
+ *   The Artemis disk image is 30 MB (61440 sectors).  The first virtio-blk
+ *   device matching PCI vendor=0x1AF4 / device=0x1042 or 0x1001 whose
+ *   sector count equals 61440 is selected.  If no exact match, the first
+ *   device found is used.
+ */
+
+#ifndef STARKERNEL_VIRTIO_BLK_H
+#define STARKERNEL_VIRTIO_BLK_H
+
+#include <stdint.h>
+#include "blkio.h"
+
+/* StarForth block size and sector ratio */
+#define VBLK_SECTOR_SIZE      512u
+#define VBLK_SECTORS_PER_BLOK 2u    /* 1 KiB Forth block = 2 × 512-byte sectors */
+#define VBLK_ARTEMIS_SECTORS  61440u /* 30 MB */
+
+/* virtio PCI vendor */
+#define VIRTIO_PCI_VENDOR_ID  0x1AF4u
+/* device IDs */
+#define VIRTIO_BLK_DEVICE_MODERN  0x1042u
+#define VIRTIO_BLK_DEVICE_LEGACY  0x1001u
+
+/* virtio-blk request types */
+#define VIRTIO_BLK_T_IN   0u
+#define VIRTIO_BLK_T_OUT  1u
+
+/* virtio-blk status codes (device → driver) */
+#define VIRTIO_BLK_S_OK     0u
+#define VIRTIO_BLK_S_IOERR  1u
+#define VIRTIO_BLK_S_UNSUPP 2u
+
+/*
+ * virtio_blk_find_artemis — locate the Artemis virtio-blk disk, initialise
+ *                           the driver, and fill in *dev_out so the caller
+ *                           can pass it to blk_subsys_attach_device().
+ *
+ * dev_out must point to a zero-initialised blkio_dev_t.
+ *
+ * Returns  0 on success.
+ * Returns -1 if no virtio-blk device was found on the PCI bus.
+ * Returns -2 on driver initialisation failure (bad BAR, queue setup, etc.).
+ */
+int virtio_blk_find_artemis(blkio_dev_t *dev_out);
+
+#endif /* STARKERNEL_VIRTIO_BLK_H */