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/xxhash64.h

@@ -0,0 +1,122 @@
+/*
+  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.
+*/
+
+/**
+ * xxhash64.h - xxHash64 Implementation (Freestanding)
+ *
+ * Fast, deterministic 64-bit hash function for capsule content addressing.
+ * No libc dependency - works in kernel context.
+ *
+ * Based on xxHash by Yann Collet (BSD-2-Clause license).
+ * https://github.com/Cyan4973/xxHash
+ */
+
+#ifndef STARKERNEL_XXHASH64_H
+#define STARKERNEL_XXHASH64_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*===========================================================================
+ * Constants
+ *===========================================================================*/
+
+/** Default seed for capsule hashing (deterministic) */
+#define XXHASH64_CAPSULE_SEED   0ULL
+
+/*===========================================================================
+ * One-Shot Hashing
+ *===========================================================================*/
+
+/**
+ * xxhash64 - Compute xxHash64 of a buffer
+ *
+ * @param data  Input data
+ * @param len   Length in bytes
+ * @param seed  Hash seed (use XXHASH64_CAPSULE_SEED for capsules)
+ * @return 64-bit hash value
+ */
+uint64_t xxhash64(const void *data, size_t len, uint64_t seed);
+
+/**
+ * xxhash64_capsule - Hash capsule payload with standard seed
+ *
+ * Convenience wrapper using XXHASH64_CAPSULE_SEED.
+ *
+ * @param data  Capsule payload bytes
+ * @param len   Payload length
+ * @return Content hash (capsule_id)
+ */
+static inline uint64_t xxhash64_capsule(const void *data, size_t len) {
+    return xxhash64(data, len, XXHASH64_CAPSULE_SEED);
+}
+
+/*===========================================================================
+ * Streaming API (for large data)
+ *===========================================================================*/
+
+/** Streaming state structure */
+typedef struct {
+    uint64_t total_len;
+    uint64_t v1;
+    uint64_t v2;
+    uint64_t v3;
+    uint64_t v4;
+    uint8_t  buffer[32];
+    uint32_t buffer_size;
+    uint64_t seed;
+} XXHash64State;
+
+/**
+ * xxhash64_reset - Initialize streaming state
+ *
+ * @param state  State to initialize
+ * @param seed   Hash seed
+ */
+void xxhash64_reset(XXHash64State *state, uint64_t seed);
+
+/**
+ * xxhash64_update - Feed data into streaming hash
+ *
+ * @param state  Streaming state
+ * @param data   Input data
+ * @param len    Length in bytes
+ */
+void xxhash64_update(XXHash64State *state, const void *data, size_t len);
+
+/**
+ * xxhash64_digest - Finalize and get hash value
+ *
+ * @param state  Streaming state
+ * @return 64-bit hash value
+ */
+uint64_t xxhash64_digest(const XXHash64State *state);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* STARKERNEL_XXHASH64_H */