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