tex.mk/noweb.mk: create _minted/ before running latexmk (minted v3 placeholder failure)

[dbosk]

Context

Sibling issue to dbosk/didactic#24. A stray _minted/ cache directory
anywhere on the TeX search path (there: a didactic clone installed in
~/texmf with in-place-built docs) makes minted v3 skip creating the
project-local _minted/; latexminted batch then crashes with
FileNotFoundError writing <project>/_minted/<hash>.highlight.minted,
and every listing in the PDF silently renders as a literal red
<MINTED> placeholder.

Observation

Any project building PDFs through the shared rules is exposed:
tex.mk's %.pdf: %.tex recipes (and noweb.mk's NOWEAVE.pdf,
which invokes latexmk directly) run latexmk without ensuring the minted
cache directory exists. Whether a given machine breaks depends on
whether some unrelated _minted/ is visible to kpathsea — a
hard-to-diagnose, machine-dependent failure.

Why it matters

One line in the shared makefiles immunises every project at once,
instead of each project rediscovering the failure and patching its own
Makefile (as dbosk/vim-noweb did with mkdir -p _minted before
latexmk).

Evidence

From the affected project's log:

(/home/dbosk/texmf/tex/latex/didactic/_minted/default.style.minted)
...
FileNotFoundError: [Errno 2] No such file or directory:
  '/home/dbosk/.vim/plugged/vim-noweb/_minted/637CE...highlight.minted'

Verified fix: creating _minted/ before latexmk makes latexminted batch write its highlight cache and all listings render again.
Environment: TeX Live 2025, minted v3.7.0, latexrestricted 0.6.2.

Suggested follow-up

Add mkdir -p for the minted cache directory to the PDF recipes —
tex.mk's %.pdf ${TEX_OUTDIR}/%.pdf: %.tex (and the .dtx variant),
and noweb.mk's NOWEAVE.pdf — before the latexmk invocation. Note
that with -output-directory=${TEX_OUTDIR} minted v3 auto-detects the
output directory, so the cache dir should be created where minted will
look for it (${TEX_OUTDIR}/_minted when an output directory is in
use, ./_minted otherwise). Harmless when minted is unused: an empty
directory.

Once minted fixes the underlying behaviour upstream (trusting a
kpsewhich-found _minted/ and then not creating the local one), the
mkdir can be dropped again.