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>

v3/src/platform/README.md

@@ -0,0 +1,20 @@
+# src/platform/
+
+Platform abstraction layer for the hosted VM build.
+
+- `alloc_host.c` — hosted (`malloc`-backed) allocator implementation of
+  `include/platform_alloc.h`.
+- `alloc_kernel.c` — a kernel-side allocator implementation of the same
+  interface. Note: `src/starkernel/vm/alloc_kernel.c` is the copy actually
+  referenced by `Makefile.starkernel`'s kernel build; this one under
+  `src/platform/` is not wired into any build target — check before relying
+  on it.
+- `platform_init.c` — platform-specific initialization dispatch.
+- `threading.c` — threading primitives backing the heartbeat background
+  thread (`HEARTBEAT_THREAD_ENABLED=1`).
+
+Subdirectories:
+- `linux/time.c` — POSIX (`clock_gettime`) timing backend.
+- `l4re/time.c` — HISTORICAL: L4Re/Fiasco.OC timing backend. L4Re was a
+  supported platform target through mid-2026; it has been removed as an
+  active target and this file is no longer wired into any build.