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>

Makefile

@@ -27,6 +27,10 @@ ISABELLE ?= isabelle
 
 # Board / architecture selection — T= picks a board (see mk/boards.mk)
 include mk/boards.mk
+# Cross-compiler: boards.mk sets CROSS_COMPILE for SBC targets
+ifdef CROSS_COMPILE
+CC := $(CROSS_COMPILE)gcc
+endif
 ARCH ?= $(shell uname -m)
 ARCH := $(strip $(ARCH))
 TARGET := fastest