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/doe_log.h

@@ -0,0 +1,61 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  Licensed under the StarForth License, Version 1.0
+*/
+
+/**
+ * starkernel/doe_log.h — DoE (Design of Experiments) CSV logger
+ *
+ * Emits per-heartbeat-tick CSV rows to the serial log, tagged with
+ * [HADES][DOE ] so they can be grepped cleanly from the QEMU log.
+ *
+ * Each row contains the 12 standard heartbeat snapshot fields plus
+ * 3 APIC timer fields from TimeTrustState:
+ *   apic_ticks, time_trust_q48, variance_q48
+ *
+ * A header row is printed automatically before the first data row.
+ *
+ * Always compiled in -- gating moved from a build-time flag
+ * (HEARTBEAT_DOE_LOG) to a runtime one (g_doe_log_enabled below) so the
+ * same build can run with or without per-tick instrumentation, toggled
+ * live via the HB-ON/HB-OFF FORTH words (register_doe_log_words()),
+ * no rebuild required.
+ */
+
+#ifndef STARKERNEL_DOE_LOG_H
+#define STARKERNEL_DOE_LOG_H
+
+#include "vm.h"
+
+/**
+ * Runtime enable flag for doe_log_tick_row() below. Defaults to 1
+ * (matches the old HEARTBEAT_DOE_LOG=1 default -- instrumentation on
+ * unless explicitly turned off). Toggled by HB-ON/HB-OFF; read, not
+ * meant to be written directly outside those two words.
+ */
+extern int g_doe_log_enabled;
+
+/**
+ * Emit one CSV row for the current heartbeat tick, unless
+ * g_doe_log_enabled is 0 (a no-op then). Pulls APIC TimeTrustState via
+ * heartbeat_state(). Prints the column header once, before the first row
+ * ever emitted -- not re-printed on every HB-ON, so a session toggled
+ * off and back on stays one continuous CSV rather than getting a second
+ * header embedded partway through.
+ *
+ * @param vm      VM instance (used for snapshot data)
+ * @param snap    Populated HeartbeatTickSnapshot from heartbeat_capture_tick_snapshot()
+ */
+void doe_log_tick_row(VM *vm, const HeartbeatTickSnapshot *snap);
+
+/**
+ * Registers HB-ON ( -- ) and HB-OFF ( -- ), which set g_doe_log_enabled
+ * to 1 and 0 respectively.
+ */
+void register_doe_log_words(VM *vm);
+
+#endif /* STARKERNEL_DOE_LOG_H */