feat(security): verify binaries with pinned Ed25519 signature on SHA256SUMS

Replace the same-origin checksum with a detached Ed25519 signature
(SHA256SUMS.sig) verified against a pinned public key before extraction,
closing #308: a compromised download mirror can no longer certify a
tampered binary. The signed manifest also binds the release version,
rejecting a forced downgrade to an older signed build.

Verification is mandatory and non-bypassable on the official download path;
custom CLOAKBROWSER_DOWNLOAD_URL mirrors keep the legacy skippable checksum.
Silent auto-update is preserved for everyone because only a constant public
key is pinned, not per-version hashes. Older installed wrappers are
unaffected — the version= line is ignored by their checksum parser.

Python uses cryptography; JS uses node:crypto. Adds tamper, downgrade, and
fail-closed tests in both languages.

cloakbrowser/config.py

@@ -25,6 +25,19 @@ PLATFORM_CHROMIUM_VERSIONS: dict[str, str] = {
     "windows-x64": "146.0.7680.177.5",
 }
 
+# ---------------------------------------------------------------------------
+# Ed25519 public keys for verifying downloaded binaries.
+#
+# Each release publishes SHA256SUMS and a detached signature SHA256SUMS.sig.
+# The wrapper verifies that signature against the keys below before trusting
+# any hash in the manifest, so the download origin alone cannot certify a
+# tampered binary. Values are base64 of the 32-byte raw public key. Multiple
+# entries are accepted to allow key rotation.
+# ---------------------------------------------------------------------------
+BINARY_SIGNING_PUBKEYS: list[str] = [
+    "MKFKwIhUcKWq5xTuNA0Ovg99njcDEcEJvmWYYhApvaU=",
+]
+
 # ---------------------------------------------------------------------------
 # Playwright default args to suppress — these leak automation signals.
 # --enable-automation: exposes navigator.webdriver = true