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/ioapic.h

@@ -0,0 +1,68 @@
+/*
+  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.
+
+ */
+
+/**
+ * ioapic.h - I/O APIC interface (amd64 only)
+ *
+ * Item 4.3.5 (FABRIC-0.md §27.5): no I/O APIC driver existed anywhere in this
+ * tree before this item. The Local APIC (apic.h) self-interrupts for the
+ * timer and needs no routing; any *legacy* IRQ (i8042 keyboard's IRQ1
+ * included) requires the I/O APIC to redirect it to a Local APIC vector.
+ */
+
+#ifndef STARKERNEL_IOAPIC_H
+#define STARKERNEL_IOAPIC_H
+
+#include <stdint.h>
+
+/**
+ * Locate and map the I/O APIC via the ACPI MADT (parsed from @p acpi_rsdp).
+ * Also captures any Interrupt Source Override entries so legacy ISA IRQs
+ * with a non-default GSI/polarity/trigger mapping route correctly.
+ *
+ * @param acpi_rsdp  BootInfo->acpi_table (RSDP), or NULL.
+ * @return 0 on success, -1 if no MADT/I/O APIC entry was found.
+ */
+int ioapic_init(void *acpi_rsdp);
+
+/**
+ * Program a redirection entry for a legacy ISA IRQ, initially masked.
+ * Honors an Interrupt Source Override for @p isa_irq if the MADT had one
+ * (different GSI, polarity, or trigger mode); otherwise uses the ISA
+ * default (GSI == isa_irq, active-high, edge-triggered) per the ACPI
+ * specification's "conforms to bus" default.
+ *
+ * @param isa_irq       Legacy IRQ number (0-15).
+ * @param vector        IDT vector to deliver (e.g. 0x21).
+ * @param dest_apic_id  Destination Local APIC ID (see apic_id()).
+ * @return 0 on success, -1 if ioapic_init() has not succeeded.
+ */
+int ioapic_route_legacy_irq(uint8_t isa_irq, uint8_t vector, uint8_t dest_apic_id);
+
+/**
+ * Clear the mask bit on a previously routed legacy IRQ's redirection entry.
+ * Call only after ioapic_route_legacy_irq() has programmed it.
+ */
+void ioapic_unmask_legacy_irq(uint8_t isa_irq);
+
+#endif /* STARKERNEL_IOAPIC_H */