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/platform_alloc.h

@@ -0,0 +1,86 @@
+/*
+  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_PLATFORM_ALLOC_H
+#define STARFORTH_PLATFORM_ALLOC_H
+
+/*
+ * platform_alloc.h - Portable memory allocation abstraction
+ *
+ * Host builds: Uses standard malloc/free
+ * Kernel builds: Uses static arena with bump allocator
+ *
+ * Usage:
+ *   void* p = sf_malloc(size);
+ *   sf_free(p);
+ *   void* p2 = sf_calloc(count, size);  // zero-initialized
+ */
+
+#include <stddef.h>
+
+/*
+ * Allocate memory block of given size.
+ * Returns NULL on failure.
+ */
+void* sf_malloc(size_t size);
+
+/*
+ * Allocate zero-initialized memory for array.
+ * Returns NULL on failure.
+ */
+void* sf_calloc(size_t count, size_t size);
+
+/*
+ * Reallocate memory block to new size.
+ * Returns NULL on failure (original block unchanged).
+ */
+void* sf_realloc(void* ptr, size_t new_size);
+
+/*
+ * Free previously allocated memory.
+ * Passing NULL is safe (no-op).
+ */
+void sf_free(void* ptr);
+
+/*
+ * Initialize the allocator (required for kernel builds).
+ * Host builds: no-op.
+ * Kernel builds: sets up the static arena.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+int sf_alloc_init(void);
+
+/*
+ * Get allocator statistics (for diagnostics).
+ */
+typedef struct sf_alloc_stats {
+    size_t total_bytes;      /* Total arena size */
+    size_t used_bytes;       /* Currently allocated */
+    size_t peak_bytes;       /* High water mark */
+    size_t alloc_count;      /* Number of allocations */
+    size_t free_count;       /* Number of frees */
+} sf_alloc_stats_t;
+
+void sf_alloc_get_stats(sf_alloc_stats_t* stats);
+
+#endif /* STARFORTH_PLATFORM_ALLOC_H */