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/kmalloc.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.
+
+ 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.
+
+ */
+
+/**
+ * kmalloc.h - Kernel heap allocator interface
+ */
+
+#ifndef STARKERNEL_KMALLOC_H
+#define STARKERNEL_KMALLOC_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+typedef struct {
+    uint64_t total_bytes;
+    uint64_t used_bytes;
+    uint64_t peak_bytes;
+    uint64_t free_bytes;
+} kmalloc_stats_t;
+
+int kmalloc_init(uint64_t heap_size_bytes);
+int kmalloc_is_initialized(void);
+void *kmalloc(size_t size);
+void *kmalloc_aligned(size_t size, size_t align);
+void kfree(void *ptr);
+kmalloc_stats_t kmalloc_get_stats(void);
+uintptr_t kmalloc_heap_base_addr(void);
+uintptr_t kmalloc_heap_end_addr(void);
+
+/* Debug/diagnostic probe -- free-list census (largest free block, count of
+   free blocks, count of allocated blocks, total blocks). Kept as permanent
+   infrastructure (FABRIC-3.md SXIII, 2026-09-10): a cheap O(n) walk over
+   the free list, useful for any future heap-shape investigation, not just
+   the one that introduced it. Not called anywhere in the normal boot path
+   today -- callers add their own log_message() call sites when debugging. */
+void kmalloc_debug_census(size_t *out_largest_free, size_t *out_free_count,
+                           size_t *out_used_count, size_t *out_total_blocks);
+
+/* Same precedent as kmalloc_debug_census() above -- sum of free-block bytes
+   and used-block bytes across the whole free list. The pairing of the two
+   (block-count census + byte-sum census) is what let FABRIC-3.md SXIII tell
+   real heap corruption apart from ordinary fragmentation: fragmentation
+   grows free_count while total_free_bytes stays flat; corruption drops
+   both together. */
+void kmalloc_debug_census_bytes(size_t *out_total_free_bytes, size_t *out_total_used_bytes);
+
+#endif /* STARKERNEL_KMALLOC_H */