ci: board-matrix build pipeline with Nexus Maven publish

Builds all boards (qemu, pc, pi4, pi5, milkv) in parallel. Installs
cross-compilers (aarch64-linux-gnu, riscv64-linux-gnu) and clang-18
for the aarch64 loader. Packs ISO (amd64/aarch64) and GPT image (riscv64).

On master push, publishes to Nexus starshipos-releases using Maven
coordinates org.f18:kernel:<version>-<sha>:<board> and
org.f18:starforth:<version>-<sha>:<board>.

CROSS_COMPILE wired into StarForth Makefile via mk/boards.mk so
make T=pi4 picks up aarch64-linux-gnu-gcc automatically.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mk/boards.mk

@@ -10,35 +10,41 @@
 ifdef T
 
 ifeq ($(T),qemu)
-  ARCH       ?= amd64
-  BOARD      := qemu
-  BOARD_NAME := QEMU x86_64
+  ARCH          ?= amd64
+  BOARD         := qemu
+  BOARD_NAME    := QEMU x86_64
+  CROSS_COMPILE :=
 
 else ifeq ($(T),pc)
-  ARCH       ?= amd64
-  BOARD      := pc
-  BOARD_NAME := x86_64 PC (native)
+  ARCH          ?= amd64
+  BOARD         := pc
+  BOARD_NAME    := x86_64 PC (native)
+  CROSS_COMPILE :=
 
 else ifeq ($(T),pi4)
-  ARCH       ?= aarch64
-  BOARD      := pi4
-  BOARD_NAME := Raspberry Pi 4 (Cortex-A72)
+  ARCH          ?= aarch64
+  BOARD         := pi4
+  BOARD_NAME    := Raspberry Pi 4 (Cortex-A72)
+  CROSS_COMPILE := aarch64-linux-gnu-
 
 else ifeq ($(T),pi5)
-  ARCH       ?= aarch64
-  BOARD      := pi5
-  BOARD_NAME := Raspberry Pi 5 (Cortex-A76)
+  ARCH          ?= aarch64
+  BOARD         := pi5
+  BOARD_NAME    := Raspberry Pi 5 (Cortex-A76)
+  CROSS_COMPILE := aarch64-linux-gnu-
 
 else ifeq ($(T),milkv)
-  ARCH       ?= riscv64
-  BOARD      := milkv
-  BOARD_NAME := Milk-V Pioneer (SG2042 RISC-V 64)
+  ARCH          ?= riscv64
+  BOARD         := milkv
+  BOARD_NAME    := Milk-V Pioneer (SG2042 RISC-V 64)
+  CROSS_COMPILE := riscv64-linux-gnu-
 
 # ── Future boards (uncomment + add flag overrides in Makefile / Makefile.starkernel) ──
 # else ifeq ($(T),zynq7020)
-#   ARCH       ?= arm
-#   BOARD      := zynq7020
-#   BOARD_NAME := Xilinx Zynq-7020 (Cortex-A9, 32-bit)
+#   ARCH          ?= arm
+#   BOARD         := zynq7020
+#   BOARD_NAME    := Xilinx Zynq-7020 (Cortex-A9, 32-bit)
+#   CROSS_COMPILE := arm-linux-gnueabihf-
 #   # NOTE: requires removing the 32-bit guardrail and adding armhf toolchain support
 
 else