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/include/physics_execution_hooks.h

@@ -0,0 +1,92 @@
+/*
+  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.
+
+ */
+
+#ifndef STARFORTH_PHYSICS_EXECUTION_HOOKS_H
+#define STARFORTH_PHYSICS_EXECUTION_HOOKS_H
+
+/*
+ * physics_execution_hooks.h - Clean abstraction for physics-driven execution hooks
+ *
+ * Purpose: Hide ALL physics machinery behind two simple function calls.
+ * The VM execution loop calls these; it doesn't know or care about:
+ *   - Heat tracking
+ *   - Decay models
+ *   - Rolling window
+ *   - Pipelining metrics
+ *   - Hotwords cache
+ *   - Profiling
+ *
+ * This keeps vm.c clean and focused on execution semantics.
+ */
+
+#include "vm.h"
+
+/*
+ * Call BEFORE executing a word.
+ * Handles: decay, heat increment, rolling window, pipelining speculation check.
+ *
+ * @param vm       The VM instance
+ * @param word     The word about to execute
+ * @param prev     The previously executed word (NULL if first in sequence)
+ */
+void physics_pre_execute(VM* vm, DictEntry* word, DictEntry* prev);
+
+/*
+ * Call AFTER executing a word.
+ * Handles: physics touch, profiler exit, DoE counter, heartbeat check.
+ *
+ * @param vm       The VM instance
+ * @param word     The word that just executed
+ */
+void physics_post_execute(VM* vm, DictEntry* word);
+
+/*
+ * Call when looking up a word (outer interpreter).
+ * Handles: decay + heat for lookup path (not inner loop).
+ *
+ * @param vm       The VM instance
+ * @param entry    The word found
+ * @param canon    Canonical dictionary entry (may differ from entry)
+ */
+void physics_on_lookup(VM* vm, DictEntry* entry, DictEntry* canon);
+
+#endif /* STARFORTH_PHYSICS_EXECUTION_HOOKS_H */