Plugin dev: add meaningful TraceObserver data usage example

[pinin4fjords]

From PR #761 review by @adamrtalbot (March 24):

The only thing I can see which is missing is using a TraceObserver and reading one of the items that originates from it, e.g. onPublish -> use file path for something.

Currently Part 5 has onFilePublish that prints the filename, but doesn't do anything meaningful with the data. A more compelling example would demonstrate using observer data for something practical, e.g.:

• Collecting published file sizes and printing a summary
• Validating published file contents
• Building a manifest of outputs

This would better show the value of trace observers beyond simple logging.

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