Preserve empty adapter metadata fields

[barlevalon]

Summary

• preserve empty artist and title fields in the canonical status<TAB>artist<TAB>title adapter record
• render title-only metadata as the title without an artist separator
• simplify the runtime path to parse adapter output once and format the parsed fields directly
• reject malformed and legacy plain-text adapter output while clearing parsed values

Tests

• bash -n nowplaying.tmux scripts/*.sh
• ./scripts/test.sh
• shellcheck nowplaying.tmux scripts/*.sh

The suite covers complete, title-only, artist-only, and empty metadata records; malformed records; Linux adapter field preservation; and end-to-end title-only rendering.

Residual risks

• The tab-delimited protocol cannot represent literal tabs, newlines, or carriage returns in metadata; such records are rejected.
• The unchanged macOS MediaRemote adapter was not exercised against a live media source in this environment.