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/vm/switch.h

@@ -0,0 +1,60 @@
+/*
+  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.
+*/
+
+/**
+ * switch.h - Cooperative VM context switch (FABRIC-3.md §XXVIII, Stage 2,
+ * 2026-09-13).
+ *
+ * sk_vm_context_switch() suspends `from`'s execution exactly where it is
+ * (mid-C-call-stack, on from's own native stack -- Stage 1) and resumes
+ * `to` -- either for the first time ever (a synthesized initial frame,
+ * entering sk_vm_switch_entry()) or exactly where `to` was itself last
+ * switched out. Returns to the caller only once something later switches
+ * back to `from` -- from from's own point of view, this call simply
+ * takes a while to return, like any blocking call.
+ *
+ * Cooperative only: nothing here is interrupt-driven yet (that's Stage
+ * 3), so this is safe to call only from ordinary FORTH word dispatch,
+ * never from ISR context.
+ */
+
+#ifndef STARKERNEL_VM_SWITCH_H
+#define STARKERNEL_VM_SWITCH_H
+
+#ifdef __STARKERNEL__
+
+struct VM;
+
+int sk_vm_context_switch(struct VM *from, struct VM *to);
+
+/* FABRIC-3.md §XXVIII Stage 3 follow-on (2026-09-14): which VM is
+ * currently physically running, tracked by the switch mechanism itself --
+ * see switch.c's own doc comment on g_switch_current_vm for why this
+ * exists instead of reusing vm_log_attributed_vm(). sk_vm_switch_set_
+ * current() seeds the initial value (whoever is running before any
+ * switch has ever happened) -- call once, at Stage 3 registration time. */
+struct VM *sk_vm_switch_current_vm(void);
+void sk_vm_switch_set_current(struct VM *vm);
+
+#endif /* __STARKERNEL__ */
+
+#endif /* STARKERNEL_VM_SWITCH_H */