Avoid nesting in event processor, do not add coupling

[ok300]

This PR proposes a way to address this review thread: #777 (comment) . This is a proposed change to #777 , so it points to that PR's branch.

More specifically, it avoids introducing (7-layer) nesting in the main event processor logic, while also avoiding the coupling counter-argument raised in the same thread.

---

AI detailed description below

The counter-argument rests on a false dilemma: that incorporating logic into Event::parse_event necessarily means teaching nexus-common about app-specific paths. It doesn't.

The root cause of the nesting is that Event::parse_event (nexus-common/src/models/event/mod.rs:70-72) treats a ParsedUri::try_from failure as InvalidEventLine:

  let parsed_uri = ParsedUri::try_from(uri.as_str()).map_err(|e| {
      EventProcessorError::InvalidEventLine(format!("Cannot parse event URI: {e}"))
  })?;

But pubky://user/pub/mapky/tags/123 is not an invalid event line — it's a valid event that pubky-app-specs doesn't recognize. By conflating "unrecognized path" with "invalid line," parse_event throws away the already-parsed event_type and uri, forcing the universal tag handler to re-parse the raw line from scratch via parse_event_line (which duplicates the split + event type validation).

The fix: Give parse_event a richer return type that distinguishes "parsed," "skipped," and "unrecognized URI":

  pub enum ParseResult {
      Parsed(Event),
      Skipped,
      UnrecognizedUri { event_type: EventType, uri: String, reason: String },
  }

nexus-common gains zero knowledge of universal tags or app-specific paths.

What this achieves:

1. Nesting drops from 7 levels to 2-3 (flat match arms)
2. parse_event_line is eliminated — no more duplicated line splitting
3. nexus-common stays free of app-specific logic
4. All universal tag business logic (try_parse_app_tag_path, handle_put, handle_del) stays unchanged in nexus-watcher
5. The retry error handling can be extracted into a small helper method mirroring the existing handle_event pattern

[tipogi]

LGTM