[TEST] Golden-vector interop test for the JS connect-widget crypto

[ScottieR]

Locks the Rust decrypt path against a ciphertext produced by canopy-admin's src/connect-widget/main.ts, so a protocol drift between the two repos (curve, HKDF info string, AEAD choice, byte layout) fails loudly.

That drift is otherwise silent: the browser keeps encrypting, this install just stops being able to decrypt.

The golden vector

Generated by importing the real encryptToInstance under Node (the module only calls main() when a DOM exists), against a throwaway recipient keypair created for the vector alone. It has never been a real instance key and decrypts only a dummy string.

• WEB_CONNECTIONS.md — how to regenerate, and why the recipient key must never be a real one
• .gitleaks.toml — allowlists the vector's literal, scoped to that exact value. Verified a different secret in the same file is still caught.

Verification

cargo test --lib web_connections → 6 passed. The JS half (connect-widget.test.js, canopy-admin) is wired into its npm test script in a companion PR.

Rescued from an uncommitted change that sat in a local worktree for five days; rebased onto current master.

🤖 Generated with Claude Code

[ScottieR]

verify is red here for a reason that isn't this PR's contents: CI's gitleaks never reads the repo's .gitleaks.toml, because gitleaks-action only honours a repo config when GITLEAKS_CONFIG names it. The allowlist entry added here works for the local pre-commit hook and is ignored by CI.

Fixed in #97 — this goes green once that merges. Nothing to change here.

(That config has carried the Privacy transaction fixture allowlist since Aug 10 without ever taking effect in CI either; this PR just happens to be the first whose finding lands on a changed line.)