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

.github/workflows/wasm-build.yml

@@ -140,6 +140,12 @@ jobs:
             !output/*.wasm.debug.wasm
           key: ${{ steps.keys.outputs.key }}
 
+      # X-1: every dep tarball fetch must carry a SHA256 pin and download_file
+      # must enforce it. Seconds, no docker, offline — runs even on cache hits
+      # so a regression is caught before it can poison the deps cache.
+      - name: Check dependency tarball pins
+        run: scripts/deps/check-pins.sh
+
       # deps (sysroot + stamps) are only needed when we must COMPILE (cache miss).
       - name: Restore deps cache
         id: deps-cache