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>

boards/zynq7020/README.md

@@ -0,0 +1,17 @@
+# Xilinx Zynq-7020 boot media
+
+The Zynq-7020's processing system is a dual Cortex-A9: ARMv7-A, 32-bit. It is the host for the
+StarForth v4 mesh (`docs/v4.0.0/JUSTIFICATION.md` §5, §8).
+
+Boot chain: BootROM, then `BOOT.BIN` (U-Boot SPL with the board's `ps7_init` DDR and clock setup),
+then U-Boot, then LithosAnanke. Everything is read from the SD card's FAT partition.
+
+Status: no image yet. `make boot_image TARGET=ZYNQ7020` stops with an error, because there is no
+ARMv7 kernel port (`kernel/src/arch/armv7`: entry code, MMU, GIC, private timer). A trial
+compile for Cortex-A9 built 147 of the 151 kernel and engine sources. The four that failed use
+`__int128` or have no ARMv7 branch: `kernel/src/crypto/fe25519.c`,
+`kernel/src/crypto/scalar25519.c`, `kernel/src/hal/hal.c` and
+`v3/src/word_source/mixed_arithmetic_words.c`.
+
+`BOOT.BIN` depends on the exact board (Zybo Z7-20, PYNQ-Z2, Arty Z7-20, ZedBoard, ...), because
+`ps7_init` holds that board's DDR timing. The board has not been chosen yet.

boards/zynq7020/board.mk

@@ -0,0 +1,11 @@
+# boards/zynq7020/board.mk -- Xilinx Zynq-7020 (dual Cortex-A9, ARMv7-A, 32-bit).
+#
+# Boot chain: BootROM -> BOOT.BIN (U-Boot SPL with the board's ps7_init DDR/
+# clock setup) -> U-Boot -> LithosAnanke, all from the SD card's FAT
+# partition. Needs the ARMv7 kernel port (kernel/src/arch/armv7), which is
+# in progress: until it exists, boot_image stops with an explicit error
+# rather than producing an image that cannot run.
+BOARD_DESC  := Xilinx Zynq-7020 (Cortex-A9, ARMv7-A, BOOT.BIN) -- ARMv7 port in progress
+BOARD_ARCH  := armv7
+BOARD_BOOT  := zynq-bootbin
+BOARD_IMAGE := lithos-zynq7020.img