Error-message quality pass — audit rendered output against PUC-Lua

[davydog187]

Tracked by plan: .agents/plans/A26-error-message-quality.md

The plan file is the source of truth for goal, scope, success criteria, and implementation notes. Comments here are for discussion; the plan is for execution.

When this work is picked up:

1. The branch named in the plan's frontmatter is created.
2. A PR is opened with Closes #<this-issue> in the body.
3. Merge closes this issue.

For strategic context, see ROADMAP.md.