Scans every .qmd under book/quarto/contents/ for footnote definitions
whose first letter is lowercase. Complements the existing heading-case
guard (see acf2ba979c) by catching terms that a global lowercasing
sweep can accidentally strip the leading capital from.
Honors an allowlist (footnote_caps_allowlist.txt) for intentional
lowercase prefixes: brand names (cuDNN, gRPC, vLLM, nn.Module),
mathematical variables (k-Center, k-Anonymity, im2col), and SI units
(pJ/MAC). The --fix flag uppercases the first letter in place while
preserving bold markers and the remainder of the body verbatim.