flush_to_file() ends with clear(), but tracing stays on and the blocks
already compiled keep the index baked into their LogIRBlock instruction.
A second flush then indexed an emptied trace_info out of bounds.
Skip indices that no longer refer to anything and say so in the log, and
give the LogIRBlock placeholder an explicit invalid index so a block that
prepare_block failed to record doesn't dump block 0 instead.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
