Retirement living standards benchmarks (PLSA) on the income presentation

[williajm]

Suggestion — a candidate roadmap item, not yet adopted into docs/planning.md §8.

Projected retirement income currently has no external anchor: the charts and the outlook card say what the plan produces, but not whether that is a little or a lot. The PLSA Retirement Living Standards (Minimum / Moderate / Comfortable, published annually with a documented basket methodology) are the standard UK yardstick for exactly this question.

Proposed shape:

• A new data file under src/glidepath/regions/uk/data/ (e.g. living_standards.toml) with the three annual income levels for a single person outside London, carrying verified_on + sources like every UK data file, strictly validated by the loader. The guard test regime applies: no hardcoded figures.
• Presentation: horizontal benchmark lines on the income composition chart (and/or the outlook card naming which band the projected income lands in — "between Moderate and Comfortable").
• Open questions for the design: single vs couple figures (couples are 9.4-gated), London vs outside-London, and whether the comparison should use net income (glidepath models net spending; the PLSA figures are expenditure baskets, so net is the right basis — worth stating explicitly in the copy).

Blocked on nothing; the only new dependency is the annually-published figures, which fit the existing verified-data pattern.