build(bios): assemble real BIOS disk image; gate SMP behind feature #11

Job	Run time
fmt	14s
miri	1m 13s
clippy	23s
deny	2m 17s
asm-verification	1m 34s
test (macos-latest, aarch64-apple-darwin)	22s
freebsd	2m 26s
test (ubuntu-latest, x86_64-unknown-linux-gnu)	26s
test (ubuntu-24.04-arm, aarch64-unknown-linux-gnu)	30s
cross (riscv64gc-unknown-none-elf)	29s
cross (x86_64-unknown-uefi)	33s
cross (aarch64-unknown-uefi)	32s
cross (loongarch64-unknown-none)	29s
size-gate	5s
qemu-smoke	1m 18s
sbom	20s
	13m 11s

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