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/src/README.md

@@ -0,0 +1,29 @@
+# src/starkernel/
+
+LithosAnanke — the bare-metal UEFI kernel that boots StarForth directly on
+hardware (amd64/aarch64/riscv64). Built only via `Makefile.starkernel`; the
+only valid acceptance test is the three-arch QEMU boot (see
+`.claude/CLAUDE.md`), never `make test`.
+
+- `kernel_main.c` — kernel entry point, driving the boot milestones
+  (console init, PMM, VMM, interrupts, timers, kmalloc heap, VM
+  bootstrap).
+- `repl.c` — kernel REPL.
+- `doe_log.c` — kernel-side DoE (Design of Experiments) metrics logging.
+
+Subdirectories:
+- `arch/{amd64,aarch64,riscv64}/` — per-architecture support (APIC/GIC/
+  PLIC interrupt controller, timers, boot/ISR assembly).
+- `boot/` — UEFI loader, ELF loading, kernel command-line parsing.
+- `capsule/` — capsule birth/run/load/validate pipeline.
+- `hal/` — hardware-abstraction-layer implementation (console,
+  framebuffer, VT100, memory, host services).
+- `hash/` — XXHash64 content-addressing implementation.
+- `math/` — kernel-build Q48.16 fixed-point arithmetic.
+- `memory/` — physical/virtual memory managers and the kernel heap.
+- `pci/` — PCI bus enumeration.
+- `virtio/` — VirtIO block device driver.
+- `vm/` — kernel VM subsystem (bootstrap, core interpreter, parity
+  logging, capsule arena).
+
+See `include/starkernel/README.md` for the corresponding headers.