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/rpi5_native_boot.h

@@ -0,0 +1,26 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+  Copyright (c) 2023–2025 Robert A. James. All rights reserved.
+  Licensed under the StarForth License, Version 1.0.
+ */
+
+/**
+ * rpi5_native_boot.h - Raspberry Pi 5 DTB->BootInfo constructor
+ * (FABRIC-3.md §IV.3 item 2).
+ *
+ * `native_rpi5_entry.S`'s `rpi5_native_start` tail-calls
+ * `rpi5_native_boot()` with the masked DTB pointer it captured. This
+ * function builds the *existing*, unmodified `BootInfo` struct
+ * (`include/starkernel/uefi.h`) from the devicetree instead of UEFI
+ * protocols, then calls the *existing*, unmodified `kernel_main()` — see
+ * this file's own `.c` for exactly what is and is not populated, and why.
+ */
+
+#ifndef STARKERNEL_RPI5_NATIVE_BOOT_H
+#define STARKERNEL_RPI5_NATIVE_BOOT_H
+
+#include <stdint.h>
+
+void rpi5_native_boot(uint64_t dtb) __attribute__((noreturn));
+
+#endif /* STARKERNEL_RPI5_NATIVE_BOOT_H */