L08 figure replacements + inline ref audit

Replace four self-made schematic figures with original published versions:
- fig:mars-vp-vs: Stähler 2021 Fig. 2A (Mars Vp/Vs profiles)
- fig:mars-wedge: Stähler 2021 Fig. 3 (Mars shadow-zone schematic)
- fig:jupiter-z: Wahl 2017 Fig. 1 (Jupiter density profiles + interior)
- fig:icegiant-interior: Helled 2020 Fig. 4 (four ice-giant structure sketches)

Defer #13 PREM (Dziewonski 1981), #15 Moon (Weber 2011), and #18 Saturn
schematic (Mankovich+Fortney 2020 not in bib): source PDFs are not locally
available. Self-made figures retained for now and flagged in memory.

Add 24 inline {numref} references throughout body prose (lecture had zero
before, the largest deficit of the cycle). Captions for the four replaced
figures rewritten to cite the source paper and figure number explicitly.

Delete obsolete generator scripts for the replaced figures.

Build verified locally, no warnings.

Author: timlichtenberg