Convert all Unicode × (U+00D7) to LaTeX $\times$ in prose, tables, and
math contexts across both volumes. Unicode × is preserved only inside
fig-alt text for accessibility screen readers. One instance inside a
plain markdown backtick code span (frameworks.qmd) was reverted to
Unicode × since LaTeX doesn't render in code spans.
Updates validate.py with a new lowercase-x-as-multiplication check and
refines the latex_adjacent warning to distinguish _str variables (safe)
from raw inline Python. Updates validate_inline_refs.py comments to
reflect the new convention. Includes the conversion script in _archive.
