Convert all logs to structured logging for consistency

PR [#328](https://github.com/google/sam/pull/328) converts the six [Mesh Event] log lines to structured Warnw/Infow so machine consumers can parse them. The remaining ~400 printf-style call sites (logger.Debugf/Infof/Warnf/Errorf) are still prose.

Convert them to the w-variants with key-value fields for consistency, so any line can be consumed programmatically without regexing message text.

Notes:

* Mechanical but large; best done in per-package chunks to keep review manageable.
* Reuse the field vocabulary started in #328(event, peer, timestamp, key) where applicable.

Start by reviewing PR #328 and inventorying the remaining logger.Debugf/Infof/Warnf/Errorf call sites, organizing the work into per-package chunks. Reuse the event, peer, timestamp, and key field vocabulary where applicable; done means the remaining printf-style call sites use structured w-variants without requiring regex parsing.

Written by the indexing model from the issue text.