[memegauste]

Parallel CI builds can start NASM or a library compilation before the kernel recipe creates build/, producing boot.asm: fatal: unable to open output file build/boot.o. The output directory is now an order-only prerequisite of every object, so all compilation waits for it without rebuilding objects when the directory timestamp changes.

Make clean safe on both fresh and already-clean checkouts, and remove the attempt to delete build/* after the directory is gone. CI now performs a clean parallel build using the Makefile defaults, validates the kernel's Multiboot header, and fails if the ISO artifact is missing.

Validation in the exact CI image (sha256:0413a96395dfe1db6ddb7311dd6212ca264c4f022c5078b85b7853787807ecd9):

• Reproduced the original NASM output-directory failure on unmodified master with make -j32 -k.
• Fresh ISO builds with make -j16 and make -j32 passed.
• Repeated make clean, incremental rebuild after touching kernel.c, and make -q passed.
• Touching the build directory did not invalidate the build.
• grub-file --is-x86-multiboot kenos.bin and nonempty ISO checks passed.
• CI shell syntax and git diff --check passed.

GitHub Actions passed for the latest commit: OSDev CI, including the clean parallel build, Multiboot validation, and ISO artifact upload.