Merge tominted-default-weave with f870419 (biber-by-default tex.mk)

[dbosk]

Context

Converting the intropy course's lecture-note decks to literate programs
(noweb contents.nw woven to contents.tex, examples tangled out). The
decks include this repo as a submodule and rely on its noweb.mk/tex.mk.

Observation

Two branches carry changes that are both needed by a literate deck but have
not been merged together:

• tominted-default-weave — makes the highlighted tominted weave the
  default in noweb.mk, ships the noweb_lexer.py copy rule, and uses the
  bracketed -R"[[filename]]" tangle form. Loads minted via
  \usepackage[minted]{noweb}.
• f870419 (on/near master) — tex.mk: select biber by default, run nonstopmode, succeed on PDF output.

A deck needs the weave/tangle machinery from the first and the biber routing
from the second at the same time.

Why it matters

With tominted-default-weave checked out but f870419 not merged in, a deck
that has no .bib still triggers a bibtexu-based .bbl rule under
latexmk -use-make, producing a noisy (though non-fatal) failing sub-make:

make[1]: *** [.../tex.mk:47: ltxobj/notes.bbl] Error 2

The PDF still builds, but the error is alarming and clutters build logs for
every biberless deck.

Evidence

• intropy commit pinning the submodule to tominted-default-weave
  (Pins makefiles at tominted-default-weave for literate decks).
• The failing-but-harmless ltxobj/*.bbl sub-make observed while building
  modules/helloworld/slides and modules/computational-thinking/slides.
• f870419 in this repo's history: "tex.mk: select biber by default, run
  nonstopmode, succeed on PDF output".

Suggested follow-up

Merge the two lineages (e.g. merge master/f870419 into
tominted-default-weave, or vice versa) so the default weave and
biber-by-default tex.mk ship together, and confirm a .bib-less deck
builds without the bibtexu sub-make error. Then re-point the intropy
submodule at the merged commit.