Add CollectLocalStreaming that uses a streaming pipeline
(walk -> tar -> compress -> encrypt -> file) via io.Pipe,
avoiding buffering the entire dataset in memory. Add DecryptStimV2
for round-trip decryption back to DataNode. Wire streaming path
into existing CollectLocal when format is "stim".
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
