rename: lumen → punktfunk, everywhere

Full project rename, decided 2026-06-10:
- Crates/binaries: punktfunk-core / punktfunk-host / punktfunk-client-rs.
- C ABI: punktfunk_* symbols, Punktfunk* types, include/punktfunk_core.h,
  PUNKTFUNK_FEATURE_QUIC guard (header regenerated; cbindgen renames updated, incl.
  PUNKTFUNK_BTN_*/PUNKTFUNK_AXIS_* wire constants).
- Protocol: punktfunk/1 — control-plane magic LMN1 → PKF1, nonce salt lmn1 → pkf1.
  WIRE BREAK: clients must be rebuilt from this revision.
- Env knobs: PUNKTFUNK_VIDEO_SOURCE / PUNKTFUNK_COMPOSITOR / PUNKTFUNK_ZEROCOPY / ….
- Host config dir: ~/.config/punktfunk (the box's dir was migrated in place — the
  persistent identity is unchanged, pinned fingerprints stay valid).
- Swift package: PunktfunkKit + PunktfunkCore.xcframework + PunktfunkConnection
  (Sources/PunktfunkClient app + tests renamed with it); build-xcframework.sh updated.
- scripts/: 60-punktfunk.rules, punktfunk-host.service; OpenAPI doc regenerated.

Also: scripts/headless/run-headless-kde.sh — full headless Plasma bringup. Root cause of
"desktop but no apps/settings" over the stream: plasmashell launched without
XDG_MENU_PREFIX=plasma-, so the launcher resolved a nonexistent applications.menu and
rendered an empty menu. The script sets the complete KDE session env (menu prefix,
KDE_FULL_SESSION, session version) and rebuilds ksycoca before starting plasmashell.

Gate: 97/97 tests, clippy -D warnings (both feature sets), fmt, C-ABI harness PASS,
zero lumen references left outside .git.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/punktfunk-host.service

@@ -0,0 +1,21 @@
+# punktfunk streaming host — systemd USER unit.
+#
+# Install:
+#   mkdir -p ~/.config/systemd/user && cp scripts/punktfunk-host.service ~/.config/systemd/user/
+#   cp scripts/host.env.example ~/.config/punktfunk/host.env   # then edit
+#   systemctl --user daemon-reload && systemctl --user enable --now punktfunk-host
+#
+# The unit assumes the compositor session (e.g. headless KWin on wayland-kde) is already up;
+# for a fully self-contained appliance, pair it with a kwin_wayland user unit it can After=.
+[Unit]
+Description=punktfunk GameStream host
+After=pipewire.service
+
+[Service]
+EnvironmentFile=%h/.config/punktfunk/host.env
+ExecStart=%h/punktfunk/target/release/punktfunk-host serve
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=default.target