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]>

docs/linux/dnf.md

@@ -0,0 +1,63 @@
+# StepForge on dnf-based Linux (Fedora / RHEL)
+
+This is the setup and packaging guide for **dnf-based** distributions. Debian,
+Ubuntu, and other apt-based systems have a separate guide:
+[apt.md](apt.md).
+
+## Install from the .rpm
+
+```bash
+sudo dnf install ./stepforge-<version>-1.<arch>.rpm
+```
+
+dnf pulls the required runtime libraries automatically (they are declared as
+`Requires`). The package installs:
+
+- the app and a fixed Electron runtime under `/opt/stepforge`,
+- the `stepforge` launcher at `/usr/bin/stepforge`,
+- a desktop entry, icons, and `.sfgz`/`.sfglt` file associations.
+
+Launch it from your application menu or run `stepforge`.
+
+### Sandbox
+
+The launcher runs **sandboxed**. On most modern kernels the Chromium
+user-namespace sandbox works out of the box; the package's `%post` also makes
+the setuid `chrome-sandbox` helper usable as a fallback. StepForge will **not**
+silently launch unsandboxed.
+
+## Install from the portable tarball
+
+The portable tarball (same one shipped for apt systems) includes the
+`/usr/bin/stepforge` launcher. Install the runtime libraries first:
+
+```bash
+bash scripts/linux/dnf/install-runtime-deps.sh
+tar -xzf stepforge_<version>_linux-x64.tar.gz
+./usr/bin/stepforge
+```
+
+## Capture capabilities on dnf systems
+
+- **X11**: full per-click capture with an accurate marker (needs `xinput`).
+- **Wayland** (Fedora's default): screen capture via the XDG Desktop Portal +
+  PipeWire; the portal asks permission once per recording. Per-click capture
+  with coordinates is not exposed by Wayland, so recording uses a global
+  hotkey or interval trigger. StepForge reports the active trigger honestly.
+
+Open Settings → Diagnostics in the app to see the detected session type,
+portal/PipeWire status, and the active capture profile.
+
+## Build the .rpm yourself
+
+```bash
+bash scripts/linux/dnf/install-build-deps.sh    # rpm-build, rpmdevtools, Xvfb, …
+nvm install && nvm use                           # pinned Node 22 (see .nvmrc)
+npm ci
+npm run package:linux:rpm                         # -> build/artifacts/*.rpm + sha256
+```
+
+The builder stages **only** runtime files (shared with the `.deb` builder via
+`packaging/linux/common/stage-runtime.sh`): the app code, a fixed Electron
+runtime, and production npm dependencies. It never copies the development
+`node_modules` and fails if `node_modules` is missing.