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>

v3/src/io.c

@@ -0,0 +1,104 @@
+/*
+  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.
+
+ 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.
+
+ */
+
+#include "io.h"
+#include "log.h"
+
+#include <string.h>
+
+/** @} */
+
+/**
+ * @brief Initialize the IO subsystem
+ * @param vm Pointer to VM instance
+ *
+ * Initializes the block device view over the unified VM memory.
+ * Zeros out the entire VM memory space.
+ */
+void io_init(VM *vm) {
+    if (!vm || !vm->memory) {
+        log_message(LOG_ERROR, "io_init: VM or VM memory not initialized");
+        return;
+    }
+
+    /* Zero the entire VM address space so blocks start clean. */
+    memset(vm->memory, 0, VM_MEMORY_SIZE);
+
+    log_message(LOG_INFO,
+                "IO subsystem initialized with %d blocks of %d bytes",
+                (int) MAX_BLOCKS, (int) BLOCK_SIZE);
+}
+
+/*
+ * @brief Write a block to VM memory
+ * @param vm Pointer to VM instance
+ * @param block_num Block number to write (0 to MAX_BLOCKS-1)
+ * @param buffer Source buffer containing block data
+ * @return 0 on success, -1 on error
+ */
+/**
+ * @brief Write a block to VM memory
+ * @param vm Pointer to VM instance
+ * @param block_num Block number to write (0 to MAX_BLOCKS-1)
+ * @param buffer Source buffer containing block data
+ * @return 0 on success, -1 on error
+ *
+ * Writes a BLOCK_SIZE chunk of data from the provided buffer into the VM's memory
+ * at the specified block number offset. The block number must be within valid range.
+ * The entire block is written atomically.
+ *
+ * @note The buffer must contain at least BLOCK_SIZE bytes of valid data
+ */
+int io_write_block(VM *vm, int block_num, const unsigned char *buffer) {
+    if (!vm || !vm->memory || !buffer) {
+        log_message(LOG_ERROR, "io_write_block: bad args");
+        return -1;
+    }
+    if (block_num < 0 || block_num >= (int) MAX_BLOCKS) {
+        log_message(LOG_ERROR, "io_write_block: invalid block number %d", block_num);
+        return -1;
+    }
+    size_t off = (size_t) block_num * (size_t) BLOCK_SIZE;
+    memcpy(&vm->memory[off], buffer, BLOCK_SIZE);
+    log_message(LOG_DEBUG, "io_write_block: wrote block %d", block_num);
+    return 0;
+}