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>

build.sh

@@ -478,18 +478,28 @@ APPRUN
     done
     [[ -f "$bd/_deps/sdl3-build/libSDL3.so" ]] && cp "$bd/_deps/sdl3-build/libSDL3.so"* "$APPDIR/usr/lib/" 2>/dev/null || true
 
-    # appimagetool
+    # appimagetool — pinned to a tagged release and SHA-256 verified before we exec it.
+    # The old "continuous" tag is a MOVING build fetched over the network and run on the release
+    # builder; a compromised/MITM'd artifact would execute here. Verify, or refuse to package.
+    local APPIMAGETOOL_URL="https://github.com/AppImage/appimagetool/releases/download/1.9.0/appimagetool-x86_64.AppImage"
+    local APPIMAGETOOL_SHA256="46fdd785094c7f6e545b61afcfb0f3d98d8eab243f644b4b17698c01d06083d1"
     local APPIMAGETOOL=""
     if command -v appimagetool &>/dev/null; then
-        APPIMAGETOOL="appimagetool"
-    elif [[ -f "$bd/appimagetool-x86_64.AppImage" ]]; then
-        APPIMAGETOOL="$bd/appimagetool-x86_64.AppImage"
+        APPIMAGETOOL="appimagetool"   # maintainer's own trusted system install
     else
-        info "Downloading appimagetool ..."
-        wget -q -O "$bd/appimagetool-x86_64.AppImage" \
-            "https://github.com/AppImage/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage"
-        chmod +x "$bd/appimagetool-x86_64.AppImage"
-        APPIMAGETOOL="$bd/appimagetool-x86_64.AppImage"
+        local at="$bd/appimagetool-x86_64.AppImage"
+        # Re-verify any cached copy too; a stale unverified download must not be trusted.
+        if [[ ! -f "$at" ]] || ! echo "${APPIMAGETOOL_SHA256}  ${at}" | sha256sum -c --status; then
+            info "Downloading appimagetool 1.9.0 (pinned) ..."
+            wget -q -O "$at" "$APPIMAGETOOL_URL"
+            if ! echo "${APPIMAGETOOL_SHA256}  ${at}" | sha256sum -c --status; then
+                err "appimagetool SHA-256 verification failed — refusing to use it"
+                rm -f "$at"
+                return 1
+            fi
+            chmod +x "$at"
+        fi
+        APPIMAGETOOL="$at"
     fi
 
     local ARCH

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..."

setup.sh

@@ -391,11 +391,26 @@ elif $SETUP_SAPLING; then
 
         SPEND_URL="https://z.cash/downloads/sapling-spend.params"
         OUTPUT_URL="https://z.cash/downloads/sapling-output.params"
+        # Consensus-critical MPC parameters with fixed, well-known SHA-256 (identical across every
+        # Zcash-family node; also pinned in scripts/build-lite-backend-artifact.sh). z.cash is
+        # plain HTTPS with no signature, so verify the digest and refuse a tampered/corrupt file.
+        SPEND_SHA256="8e48ffd23abb3a5fd9c5589204f32d9c31285a04b78096ba40a79b75677efc13"
+        OUTPUT_SHA256="2f0ebbcbb9bb0bcffe95a397e7eba89c29eb4dde6191c339db88570e3f3fb0e4"
 
-        curl -fSL -o "$PARAMS_DIR/sapling-spend.params"  "$SPEND_URL"  && \
-            ok "Downloaded sapling-spend.params"
-        curl -fSL -o "$PARAMS_DIR/sapling-output.params" "$OUTPUT_URL" && \
-            ok "Downloaded sapling-output.params"
+        if curl -fSL -o "$PARAMS_DIR/sapling-spend.params" "$SPEND_URL" \
+           && echo "${SPEND_SHA256}  $PARAMS_DIR/sapling-spend.params" | sha256sum -c --status; then
+            ok "Downloaded + verified sapling-spend.params"
+        else
+            rm -f "$PARAMS_DIR/sapling-spend.params"
+            err "sapling-spend.params download or SHA-256 verification failed — not installed"
+        fi
+        if curl -fSL -o "$PARAMS_DIR/sapling-output.params" "$OUTPUT_URL" \
+           && echo "${OUTPUT_SHA256}  $PARAMS_DIR/sapling-output.params" | sha256sum -c --status; then
+            ok "Downloaded + verified sapling-output.params"
+        else
+            rm -f "$PARAMS_DIR/sapling-output.params"
+            err "sapling-output.params download or SHA-256 verification failed — not installed"
+        fi
     fi
 else
     skip "Sapling params not found (use --sapling to download, or they'll be extracted at runtime from embedded builds)"