"chosen" LFX match status has no consumer — implement the promised fill-only canonical promotion

## Problem

The multi-profile LFX enrichment ranks a handle's profiles and labels the best one `chosen` and the rest `duplicate` (`lfx/enricher.go`). The spec (`specifications/lfx-integration.md`) describes the chosen profile being promoted onto the maintainer record fill-only (populate empty fields, never overwrite curated data) — but no code consumes `chosen` today. The status is written and displayed as a badge, and that's it.

## Options

1. Implement the fill-only promotion as specified: when a `chosen` observation exists and the maintainer's corresponding fields are empty, copy them over during enrichment.
2. Or adjust the spec/UI wording so `chosen` is documented as informational ranking only.

Option 1 is the original intent; it just never landed.

## Origin

Surfaced during PR #157 review triage (non-blocking).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start in lfx/enricher.go and read the corresponding section of specifications/lfx-integration.md to trace how chosen observations are produced and how maintainer fields are represented. Implement the original fill-only behavior: chosen values populate empty corresponding fields without overwriting curated data, and verify the enrichment behavior with the repository's relevant tests.

Written by the indexing model from the issue text.