Add post-build script that produces ratdeck-merged.bin with bootloader,
partitions, boot_app0, and firmware at correct offsets. CI now attaches
the merged binary as a standalone release asset alongside the existing
multi-part ZIP.
Package bootloader, partitions, boot_app0, and firmware as separate
files in a ZIP with manifest.json specifying flash addresses and
settings. This enables the web flasher to flash each part at its
correct address individually, matching proven web flasher patterns.
