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/ser5/README.md

@@ -0,0 +1,14 @@
+# Beelink SER5 boot media
+
+`make boot_image TARGET=SER5` builds `build/boards/ser5/lithos-ser5.img`: a GPT disk with one
+FAT32 EFI System Partition. Write it to a USB stick with `dd`. The partition holds:
+
+- `EFI/BOOT/BOOTX64.EFI`: the amd64 monolithic loader, with the whole kernel embedded. Firmware
+  finds it through the UEFI removable-media path, so no boot entry has to be created.
+- `startup.nsh`: runs the loader if the firmware drops into the UEFI shell.
+- `starforth.cfg`: only present when `KERNEL_ARGS="..."` is given.
+
+Nothing in the image is SER5-specific. It follows the generic UEFI/ACPI path in FABRIC-3.md §III.
+
+Status: booted in QEMU (q35 + OVMF, from this image as its own disk) through POST, with Stadium
+conservation exact (sum 65536 = `Q48_ONE`). Not yet booted on the SER5 itself.

boards/ser5/board.mk

@@ -0,0 +1,10 @@
+# boards/ser5/board.mk -- Beelink SER5 (AMD Ryzen, x86-64, UEFI/ACPI).
+#
+# Generic standards-compliant UEFI path (FABRIC-3.md §III): nothing in the
+# image is SER5-specific. The monolithic loader embeds the whole kernel, so
+# the image is a GPT disk with one FAT32 EFI System Partition holding
+# EFI/BOOT/BOOTX64.EFI. Write it to a USB stick with dd.
+BOARD_DESC  := Beelink SER5 (AMD Ryzen, x86-64, UEFI)
+BOARD_ARCH  := amd64
+BOARD_BOOT  := uefi-esp
+BOARD_IMAGE := lithos-ser5.img