Zodia tracked zero retrograde-motion awareness anywhere in the
codebase — a real gap for an astrology app, since retrograde Mercury
is one of the most commonly discussed astrological concepts. Adds
zodia_core::is_retrograde(planet, jdn), detected numerically (sign of
the one-day-back longitude delta, handling 0/360 wraparound) since the
simplified ephemeris model has no per-body analytic velocity term.
Verified against real-world retrograde patterns before writing
assertions: Mercury shows ~69 retrograde days/year in this model
(matches its real ~3-4 annual retrograde periods), outer planets
substantially more (Earth laps them yearly), Sun/Moon never (a real
geocentric-astrology invariant, not a model artifact).
Surfaced as a classical "℞" glyph next to any retrograde planet on the
Chart tab's placement rows.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
