Add production .rpm packaging and dnf setup; share runtime staging

Phase 3 of the improvement plan (PR 9 of the sequence): the dnf/Fedora
packaging half, in separate Linux-specific files, mirroring the apt/.deb work.

- packaging/linux/common/stage-runtime.sh: shared runtime-only payload staging
  extracted from the .deb builder so the two package formats never drift. It
  copies app code + a fixed Electron runtime + production npm deps (npm ls
  --omit=dev), guards against dev-dep leaks, and fails without node_modules.
  The .deb builder now delegates to it.
- packaging/linux/fedora/stepforge.spec: runtime Requires (nss, nspr, gtk3,
  …), Recommends (xinput, portal, pipewire), %license, and a %post that makes
  chrome-sandbox setuid and refreshes desktop/MIME/icon caches. No compilation
  — it packages the prebuilt BuildRoot.
- packaging/linux/fedora/package.sh: stages the shared payload, substitutes
  version/maintainer into the spec, and runs rpmbuild against the prebuilt
  BuildRoot with detected arch. Requires rpmbuild + node_modules; emits an
  .rpm + sha256.
- scripts/linux/dnf/install-runtime-deps.sh and install-build-deps.sh:
  separate runtime vs build dependency sets for dnf (runtime installs no build
  tools).
- docs/linux/dnf.md: Fedora/RHEL install + build guide, distinct from apt.md.

Tests: packaging-linux.test.js gains Fedora/dnf structural checks (spec
Requires + MPL-2.0 + %license + sandbox setup, builder shares staging +
requires rpmbuild, dnf build/runtime dep separation, shared staging never
copies the whole dev tree). tests/integration/linux/package-rpm.test.sh builds
a real .rpm and asserts runtime-only contents (honest skip when rpmbuild is
absent, as on this apt host).

Verified: 13 packaging unit tests pass; the refactored .deb builder still
produces a valid runtime-only package (integration test green); the .rpm
integration test skips cleanly where rpmbuild is unavailable.

Co-Authored-By: Claude Fable 5 <[email protected]>

scripts/linux/dnf/install-build-deps.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Install the BUILD toolchain for producing StepForge packages on dnf-based
+# systems (Fedora/RHEL). For DEVELOPERS/packagers only; never shipped inside
+# the end-user package.
+set -euo pipefail
+
+if ! command -v dnf >/dev/null 2>&1; then
+  echo "This script is for dnf-based systems (Fedora/RHEL)." >&2
+  exit 1
+fi
+
+SUDO=""
+if [ "$(id -u)" -ne 0 ]; then SUDO="sudo"; fi
+
+PACKAGES=(
+  rpm-build rpmdevtools   # build the .rpm
+  desktop-file-utils      # validate the .desktop entry
+  ca-certificates         # npm ci over https
+  xorg-x11-server-Xvfb    # headless smoke test under Xvfb
+)
+
+echo "Installing StepForge build dependencies via dnf..."
+$SUDO dnf install -y "${PACKAGES[@]}"
+
+cat <<'MSG'
+Done. Also install the pinned Node toolchain (see .nvmrc — Node 22.12+):
+  nvm install && nvm use     # or another Node 22 LTS install method
+Then, from the repo root:
+  npm ci
+  npm run package:linux:rpm
+MSG