Pin CI pip installs by hash and update KaTeX/DOMPurify

The pip installs in check_amalgamation.yml, ubuntu.yml (Meson) and
semgrep.yml now use --require-hashes with requirements files compiled by
pip-compile --generate-hashes (Python 3.14, as on ubuntu-26.04). The
astyle and generate_natvis requirements move to requirements.in, and
Meson and Semgrep get theirs in .github/requirements, which Dependabot
now watches. check_amalgamation.yml installs the natvis requirements
from the PR tree, as it already does for astyle.

The Mermaid checker overrides KaTeX to 0.18.11 (GHSA-238p-pmpm-9mq7;
Mermaid 11 still requires ^0.16.47) and updates DOMPurify to 3.4.17
(GHSA-cg2g-xh64-v4q7, GHSA-mrv2-3pqr-j5jr, GHSA-c6gr-qvv4-5v3f).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tools/astyle/requirements.in

@@ -0,0 +1,4 @@
+# astyle is deliberately pinned: newer versions reformat the code, and this version
+# defines the formatting that `make amalgamate` produces and check_amalgamation.yml enforces.
+# Dependabot is told to ignore it in .github/dependabot.yml.
+astyle==3.4.13