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>

.github/requirements/meson.in

@@ -0,0 +1 @@
+meson==1.12.1

.github/requirements/meson.txt

@@ -0,0 +1,10 @@
+#
+# This file is autogenerated by pip-compile with Python 3.14
+# by the following command:
+#
+#    pip-compile --generate-hashes --no-emit-index-url --output-file=.github/requirements/meson.txt --strip-extras .github/requirements/meson.in
+#
+meson==1.12.1 \
+    --hash=sha256:930bc7542cbd9f57009e182fd014eba48cf1a0180a7b9006d2d32d8f168d8b02 \
+    --hash=sha256:ab0a6ca09f8ef70c564c8241fb5a23957886a0b53fb58412b5e07eaf07dba743
+    # via -r .github/requirements/meson.in

.github/requirements/semgrep.in

@@ -0,0 +1 @@
+semgrep==1.168.0