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/include/math_portable.h

@@ -0,0 +1,91 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+      https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+ StarForth — Steady-State Virtual Machine Runtime
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  This file is part of the StarForth project.
+
+  Licensed under the StarForth License, Version 1.0 (the "License");
+  you may not use this file except in compliance with the License.
+
+  You may obtain a copy of the License at:
+       https://github.com/star.4th@proton.me/StarForth/LICENSE.txt
+
+  This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND,
+  express or implied, including but not limited to the warranties of
+  merchantability, fitness for a particular purpose, and noninfringement.
+
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+ */
+
+/*
+ * math_portable.h
+ *
+ * Portable C99 math functions in Q48.16 64-bit fixed-point format.
+ * Suitable for embedded environments with no libm
+ * (HISTORICAL: also L4Re, through mid-2026; removed as an active target).
+ * All arithmetic is integer-only for maximum performance.
+ *
+ * Q48.16 Format:
+ *   - 48-bit signed integer + 16-bit fractional part
+ *   - Precision: 2^-16 ≈ 0.0000153 (nanoseconds)
+ *   - Range: ±140 trillion ns (≈ 4.4 years)
+ *
+ * Functions:
+ *   sqrt_q48(x)  - Square root of Q48.16 value via Newton-Raphson
+ *   erf_q48(x)   - Error function of Q48.16 value (dimensionless)
+ */
+
+#ifndef MATH_PORTABLE_H
+#define MATH_PORTABLE_H
+
+#include <stdint.h>
+
+#define Q48_SCALE (1LL << 16)  /* 65536 */
+
+/**
+ * Square root in Q48.16 fixed-point
+ * Input: x in Q48.16 format
+ * Output: sqrt(x) in Q48.16 format
+ * Uses Newton-Raphson: x_{n+1} = (x_n + a/x_n) / 2
+ * Converges in ~10-15 iterations
+ * Returns 0 for negative inputs (guard against domain error)
+ */
+int64_t sqrt_q48(int64_t x);
+
+/**
+ * Error function in Q48.16 fixed-point
+ * Input: x in Q48.16 format (dimensionless Z-score)
+ * Output: erf(x) in Q48.16 format (range: -Q48_SCALE to +Q48_SCALE)
+ * Uses Abramowitz & Stegun approximation (7.1.26)
+ * Maximum error: 1.5e-7 relative to floating-point erf
+ * Properties:
+ *   - erf(0) = 0
+ *   - erf(-x) = -erf(x) (odd function)
+ *   - erf(x) approaches ±Q48_SCALE for large |x|
+ */
+int64_t erf_q48(int64_t x);
+
+#endif /* MATH_PORTABLE_H */