Simplify reproducible builds: single version tag, add binary tarball + AppImage outputs

- Replace docker-reproducible.yml with reproducible.yml which produces
  three artifacts per arch: Docker image, binary tarball, and AppImage
- Use a single multi-arch index digest in Dockerfile.reproducible as the
  sole version tag to maintain; Makefile and CI no longer carry their own
  per-arch image references
- Add packaging/appimage/ template (AppRun, .desktop, lighthouse.svg)

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

packaging/appimage/AppRun

@@ -0,0 +1,9 @@
+#!/bin/sh
+# AppRun - AppImage entry point for Lighthouse
+# Resolves the real location of the AppImage and executes the bundled binary.
+set -e
+
+SELF=$(readlink -f "$0")
+HERE=$(dirname "$SELF")
+
+exec "$HERE/usr/bin/lighthouse" "$@"