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/include/starkernel/rpi5_mailbox.h

@@ -0,0 +1,68 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+  Copyright (c) 2023–2025 Robert A. James. All rights reserved.
+  Licensed under the StarForth License, Version 1.0.
+ */
+
+/**
+ * rpi5_mailbox.h - VideoCore mailbox property-interface framebuffer
+ * setup, for the native (non-UEFI) Raspberry Pi 5 boot path
+ * (FABRIC-3.md §IV.3 item 3).
+ *
+ * Register layout, message/tag format, and property-tag IDs confirmed
+ * against multiple sources before writing `rpi5_mailbox.c` — see that
+ * file's own doc comment for exactly which, and what (if anything)
+ * remains unverified against real hardware (not in hand until
+ * 2026-09-17; this driver has never run on real silicon).
+ */
+
+#ifndef STARKERNEL_RPI5_MAILBOX_H
+#define STARKERNEL_RPI5_MAILBOX_H
+
+#include <stdint.h>
+
+/* Mirrors uefi.h's FramebufferInfo exactly -- populated by
+ * rpi5_mailbox_get_framebuffer() the same shape UEFI GOP already
+ * populates it, so console.c/vt100.c/framebuffer.c need no changes at
+ * all for this path. Not #include-ing uefi.h here (that header is a
+ * large UEFI-protocol grab-bag; this driver only needs this one
+ * struct's shape) -- callers that already have a `FramebufferInfo*`
+ * (from uefi.h) can pass it directly, since the two struct
+ * definitions are kept in exact field-for-field sync by convention.
+ */
+typedef struct {
+    void*    base;
+    uint64_t size;
+    uint32_t width;
+    uint32_t height;
+    uint32_t pixels_per_scanline;
+    uint32_t pixel_format; /* 0 = RGB, matches uefi.h's
+                             * PixelRedGreenBlueReserved8BitPerColor --
+                             * this driver always requests RGB pixel
+                             * order explicitly (tag 0x00048006), never
+                             * leaves it at hardware/firmware default. */
+} Rpi5FramebufferInfo;
+
+/**
+ * @brief Request a framebuffer from the VideoCore firmware via the
+ * mailbox property interface, at the requested resolution/depth.
+ *
+ * Sends one buffer with all six setup tags (physical size, virtual
+ * size, depth, pixel order, virtual offset, allocate) plus a
+ * get-pitch tag, in one request/response round trip.
+ *
+ * @param dtb     Devicetree blob (passed to `rpi5_mailbox_base()`).
+ * @param width   Requested physical+virtual width, pixels.
+ * @param height  Requested physical+virtual height, pixels.
+ * @param bpp     Requested bits per pixel (32 is the only depth this
+ *                driver has been designed against; others are not
+ *                refused outright but are unverified).
+ * @param out     Populated on success; untouched on failure.
+ * @return 0 on success, negative on failure (mailbox node not found
+ *         in the DTB, VC firmware rejected the request, or a response
+ *         tag came back with an unexpected size).
+ */
+int rpi5_mailbox_get_framebuffer(const void* dtb, uint32_t width, uint32_t height,
+                                  uint32_t bpp, Rpi5FramebufferInfo* out);
+
+#endif /* STARKERNEL_RPI5_MAILBOX_H */