packaging(deb): add armv7l to build-matrix.sh (QEMU binfmt + debian:12 armhf rootfs)

[satwareAG-ironMike]

Context

Add 32-bit ARM (armv7l / Debian armhf) to the Debian build matrix. This covers Raspberry Pi 4/5 and other ARM SBCs running Debian/Ubuntu.

Build uses QEMU binfmt_misc to cross-compile inside an armhf Docker rootfs on an x86_64 host.

Acceptance Criteria

• packaging/debian/build-matrix.sh adds armv7l to the arch list
• Build script detects armv7l and uses debian:12-armhf rootfs via QEMU
• QEMU binfmt registered: docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
• FB5 client downloaded for linux-arm (or built from source if no arm binary exists)
• .deb produced for armhf: php8.4-firebird_13.0.0-1_armhf.deb
• Package installs and loads on Raspberry Pi 4 (or QEMU emulated armhf container)

Dependencies

• Welcome to php-firebird Discussions! #2 (build.sh) — single build script
• Create Docker-based PHP Firebird dev environment setup #3 (build-matrix.sh) — matrix runner to extend

Suggested Test Plan

1. Run packaging/debian/build-matrix.sh --arch armv7l --php-versions 8.4 --distros bookworm
2. Verify armhf.deb produced
3. dpkg --print-architecture in armhf container — shows armhf
4. dpkg -i *.deb && php -m | grep firebird — loads on armhf