v1.4.21 — Add a Π vs Σ comparison table
Chapter 1 §5 discussed Π-types and Σ-types at length in separate
sections but never gathered their notation, readings, and special cases
into one side-by-side comparison. Added a recap table right after both
constructs are introduced (notation, what each generalizes, categorical
reading, Lean surface syntax, the ∀/∃ logic special case, the
witness-extraction asymmetry between Sigma and Exists, and each
construct’s first worked example earlier in the section).
Caught and fixed a table-rendering bug while writing it: an empty first
header cell broke Pandoc’s Markdown-to-LaTeX table conversion (the
header row lost its \toprule/content structure entirely). Giving the
column a real header (“Aspect”) fixed it — verified with a full PDF
rebuild.