findings X-1: pin every dependency tarball fetch to a SHA256 and enforce it

security-audit-v3 #15. download_file already had a verify branch; no caller
used it and every *_SHA256 in versions.sh was a commented placeholder, so a
tampered mirror tarball flowed straight into configure/make and the shipped
WASM.

- versions.sh: 13 pins (cross-checked against Homebrew/Buildroot/nixpkgs/
  FreeBSD/vcpkg/boost.org/curl PGP; glm .zip is TOFU), boost/curl/libgit2
  versions moved beside their pins.
- all 13 download_file call sites pass "${NAME_SHA256}".
- download_file refuses an empty or malformed pin (PCBJAM_ALLOW_UNPINNED=1
  to bootstrap a new dep); file_sha256 prefers sha256sum, falls back to shasum.
- scripts/deps/check-pins.sh: static 3-arg check + offline file:// enforcement
  test; runs in wasm-build.yml before the deps cache, on cache hits too.

Expect one cold --build-deps run: the deps-cache key hashes versions.sh.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GcsgJZ77bhZatLAVU8R84H

scripts/deps/build-boost.sh

@@ -48,7 +48,7 @@ if [ ! -d "${BOOST_DIR}" ]; then
     BOOST_ARCHIVE="boost_${BOOST_VERSION_UNDERSCORE}.tar.gz"
 
     # Download and extract
-    download_file "${BOOST_URL}" "${BOOST_ARCHIVE}"
+    download_file "${BOOST_URL}" "${BOOST_ARCHIVE}" "${BOOST_SHA256}"
     tar -xzf "${BOOST_ARCHIVE}"
     rm "${BOOST_ARCHIVE}"
 fi