Test streaming

[braceal]

Gets the streaming DDWE workflow (DDWEStreamThinker) running end-to-end on Vista, and adds a complete streaming example plus setup docs.

Four bugs found while debugging this branch were split out and merged separately: #48 (WeightedEnsemble.metadata default), #49 (inverted use_stale_model gating), #50 (print_errors on null failure_info), #51 (unbounded re-train history). main has been merged back in, so what remains here is only the streaming feature work.

Stream configuration

ProxyStreamConfig gains two validators (workflows/stream.py):

• redis_host accepts the sentinel value hostname, resolved to address_by_hostname() at load time. This is what lets one config file work unchanged across compute nodes, where the broker address is not known until the job lands.
• store_config requires a hostname when the connector kind is redis, and accepts the same hostname sentinel. Previously a missing hostname surfaced later as an opaque connector error; it now fails at config load with a message that names the fix.

StreamProducer(publisher, stores={topic: store}) now passes stores as a keyword, per proxystore/proxystore#641.

proxystore is installed as proxystore[redis].

Proxy lifetime for streamed model weights

The training producer sends its TrainResult with evict=False. The default eviction-on-consume is wrong here: the thinker holds the model across iterations and hands it to every inference task until a newer one arrives, so evicting at consume time pulls the object out from under in-flight inference.

Memory is still reclaimed — train_stream_processor explicitly evicts the previous proxy's key when it is replaced, which is now logged on both sides of the call.

Example and docs

• New examples/openmm_ntl9_ddwe_stream/ — config, CVAE config, NTL9 basis state, reference structure, and a pretrained checkpoint for a runnable streaming NTL9 DDWE run.
• README gains a Running with streaming section: the stream_config block with a commented FileConnector alternative, how to start and kill a background redis server, and how to watch its memory usage. The install instructions add conda install anaconda::redis.

Notes for reviewers

• Much of the remaining diff is print / logger.info instrumentation added while debugging the stream path. Worth deciding what should stay as permanent logging versus what should be dropped before merge.
• examples/openmm_ntl9_ddwe_stream/checkpoint-epoch-100.pt is a 13 MB binary committed directly to the repo. Flagging in case it belongs somewhere out-of-tree.
• Inference failure now logs at error rather than warning, since it stops the workflow.
• The commit history is debug iteration (debug, logs, config, …) and would be better squashed on merge.