First release since 0.6.44 - 0.6.45 through 0.6.49 were development
rungs that never reached PyPI or crates.io.

PD1-PD4: the differential-phase reference index follows the wavelength
(0.226 rad of silent error on a dispersive ambient, fixed); PDts/PDtp
as first-class compute() observables; per-lambda reference arrays at
the FFI and Python doors with a permanent scalar guard; a decided,
pinned GD/GDD convention over Dphi.

Reviewed twice against docs/implementation_plan_pd.md; both rounds are
recorded in docs/implementation_review_pd.md.