Build a deterministic QEMU smoke test for riscv64 boot #18

Context: We already have buck2 run //sys:loader-qemu. Need a deterministic "does a boot still work?" signal on every PR.

Scope:

1. Add sys/kernel/src/tests/smoke.rs that runs a fixed init sequence, logs a sentinel string to UART, and calls sbi::shutdown() with exit code 0.
2. Add build/qemu_test.bzl macro wrapping qemu-system-riscv64 -nographic -serial stdio, capturing stdout, grepping for the sentinel, timing out after 60s.
3. Wire into test-qemu-riscv64 CI job.
4. Target <2 minute wall time.

Acceptance: CI job boots the kernel to sentinel on every PR.