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>

mk/Kconfig.mk

@@ -1,5 +1,5 @@
 # ==============================================================================
-# mk/Kconfig.mk — shared Kconfig bridge for Makefile and Makefile.starkernel
+# mk/Kconfig.mk — shared Kconfig bridge for Makefile and kernel/Makefile
 # ==============================================================================
 #
 # Included by both build systems, after each has finished resolving its own
@@ -101,7 +101,7 @@ savedefconfig: $(KCONFIG_CONF)
 # dispatching any -jN jobs" behavior fully serializes "regenerate config"
 # before "compile anything" -- for free, with no hand-maintained
 # prerequisite list. Deliberately not the include/version.h-style
-# order-only pattern (Makefile.starkernel:49-53/656-658), which needed its
+# order-only pattern (kernel/Makefile:49-53/656-658), which needed its
 # own special-cased -j/clean workaround once already.
 #
 # Guarded on the config file already existing: until a developer opts in via

mk/README.md

@@ -1,7 +1,7 @@
 # mk/
 
 - `Kconfig.mk` — shared Kconfig bridge included by both `Makefile` and
-  `Makefile.starkernel`. Each Makefile canonicalizes its own `ARCH` and
+  `kernel/Makefile`. Each Makefile canonicalizes its own `ARCH` and
   sets `KCONFIG_ARCH_DIR` before including this file, since the hosted
   and kernel builds map `aarch64` to different object-directory names.
   Config lives per-architecture under `build/$(KCONFIG_ARCH_DIR)/` rather