L01: connect every figure to body prose via inline {numref} refs

Audit found that 24 of 25 figures in L01 had no body-prose reference
(only family-portrait carried one inside another caption). Added a
{numref} reference for each figure at the appropriate point in the
flowing text, with short bridging sentences for the orphan cluster
of exoplanet figures (cumulative tally, mass-period, radius gap, HL
Tau ALMA, HD 209458 b transit) which previously had no introductory
prose at all.

No figures replaced or removed; build passes.

Author: timlichtenberg