trace_model_path dead-ended on KJNodes GetNode virtual links and fell back
to the first entry in MODELS, which could be a stale checkpoint name
resurrected from the process-lifetime node_cache — images ended up with a
wrong or missing Hashes.model.
- SetNodeExtractor now records MODEL passthrough as a model_variable entry
(variable name -> source node id), cached per node like other metadata
- trace_model_path resolves GetNode variable references through those
entries instead of giving up (last SetNode wins, per KJNodes semantics)
- _fill_missing_metadata validates cached checkpoint names against the
node's current widget values and rebuilds stale entries from the current
inputs instead of resurrecting the previous model
