bug: params read by layouts but missing from default params.toml (custom_css, pages.show_date, show_date)

[davidsneighbour]

Layouts read ananke.custom_css, ananke.pages.show_date, and ananke.show_date, but none of them are present in the theme's config/_default/params.toml. Either add them with documented defaults or remove the reads.

This blocks accurate "dummy-safe" documentation: we cannot document params that aren't shipped as defaults.

Migrated from scratch/todo.documentation.md §4 (theme-side follow-ups).