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/src/vm/vm_internal.h

@@ -0,0 +1,99 @@
+/*
+  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 VM_INTERNAL_H
+#define VM_INTERNAL_H
+
+#include "../../../v3/include/vm.h"
+#include "../../../v3/include/vm_host.h"
+
+#ifndef PARITY_MODE
+#define PARITY_MODE 0
+#endif
+
+#ifndef SK_PARITY_DEBUG
+#define SK_PARITY_DEBUG 0
+#endif
+
+/* HISTORICAL: this condition also excluded L4RE_TARGET (`&&
+ * !defined(L4RE_TARGET)`) -- L4Re had no pthreads. L4Re support has been
+ * removed as an active target (it was hosted-only anyway; this kernel
+ * file never actually saw L4RE_TARGET defined). */
+#if HEARTBEAT_THREAD_ENABLED
+#include <pthread.h>
+#define HEARTBEAT_HAS_THREADS 1
+#else
+#define HEARTBEAT_HAS_THREADS 0
+#endif
+
+#define HEARTBEAT_DECAY_BATCH 64u
+
+typedef struct HeartbeatWorker {
+#if HEARTBEAT_HAS_THREADS
+    pthread_t thread;
+#endif
+    uint64_t tick_ns;
+    int running;
+    int stop_requested;
+} HeartbeatWorker;
+
+const VMHostServices* vm_default_host_services(void);
+const VMHostServices* vm_host(const VM *vm);
+void* vm_host_alloc(VM *vm, size_t size, size_t align);
+void* vm_host_calloc(VM *vm, size_t n, size_t size);
+void vm_host_free(VM *vm, void *ptr);
+#ifndef __STARKERNEL__
+void vm_reset_hosted_fake_ns(VM *vm);
+#endif
+
+uint64_t vm_monotonic_ns(VM *vm);
+unsigned vm_get_base(const VM* vm);
+void vm_set_base(VM* vm, unsigned b);
+uint32_t heartbeat_snapshot_index_load(const volatile uint32_t *ptr);
+void heartbeat_snapshot_index_store(volatile uint32_t *ptr, uint32_t value);
+void heartbeat_publish_snapshot(VM *vm);
+void vm_heartbeat_run_cycle(VM *vm);
+#if HEARTBEAT_HAS_THREADS
+void* heartbeat_thread_main(void *arg);
+#endif
+
+#endif /* VM_INTERNAL_H */