fix(build): verify pinned build inputs (appimagetool, Sapling params)

Release-pipeline supply-chain hardening from the audit (F15-3/4, F15-6):

- appimagetool: stop fetching the moving "continuous" AppImageKit tag and
  running it unverified on the release builder. Pin appimagetool 1.9.0 and
  verify its SHA-256 (46fdd785…83d1) before exec, refusing to package on
  mismatch; a system-installed appimagetool (maintainer's own) is still used
  as-is. Applied in both build.sh and scripts/create-appimage.sh.
- setup.sh --sapling: the z.cash param download is plain HTTPS with no
  signature; verify both params against their fixed canonical SHA-256 (the
  same values already pinned in build-lite-backend-artifact.sh) and delete
  the file on mismatch.

All three scripts pass bash -n.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/create-appimage.sh

@@ -24,18 +24,27 @@ if [ ! -f "${BUILD_DIR}/bin/ObsidianDragon" ]; then
     exit 1
 fi
 
-# Check for appimagetool
+# Check for appimagetool — pinned to a tagged release and SHA-256 verified before we exec it.
+# The old "continuous" tag is a moving, unverified network download that runs on the release
+# builder; verify it or refuse to package.
+APPIMAGETOOL_URL="https://github.com/AppImage/appimagetool/releases/download/1.9.0/appimagetool-x86_64.AppImage"
+APPIMAGETOOL_SHA256="46fdd785094c7f6e545b61afcfb0f3d98d8eab243f644b4b17698c01d06083d1"
 APPIMAGETOOL=""
 if command -v appimagetool &> /dev/null; then
-    APPIMAGETOOL="appimagetool"
-elif [ -f "${BUILD_DIR}/appimagetool-x86_64.AppImage" ]; then
-    APPIMAGETOOL="${BUILD_DIR}/appimagetool-x86_64.AppImage"
+    APPIMAGETOOL="appimagetool"   # maintainer's own trusted system install
 else
-    print_status "Downloading appimagetool..."
-    wget -q -O "${BUILD_DIR}/appimagetool-x86_64.AppImage" \
-        "https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage"
-    chmod +x "${BUILD_DIR}/appimagetool-x86_64.AppImage"
-    APPIMAGETOOL="${BUILD_DIR}/appimagetool-x86_64.AppImage"
+    AT="${BUILD_DIR}/appimagetool-x86_64.AppImage"
+    if [ ! -f "$AT" ] || ! echo "${APPIMAGETOOL_SHA256}  ${AT}" | sha256sum -c --status; then
+        print_status "Downloading appimagetool 1.9.0 (pinned)..."
+        wget -q -O "$AT" "$APPIMAGETOOL_URL"
+        if ! echo "${APPIMAGETOOL_SHA256}  ${AT}" | sha256sum -c --status; then
+            print_error "appimagetool SHA-256 verification failed — refusing to use it"
+            rm -f "$AT"
+            exit 1
+        fi
+        chmod +x "$AT"
+    fi
+    APPIMAGETOOL="$AT"
 fi
 
 print_status "Creating AppDir structure..."