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>

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)"