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/host.env.example

@@ -1,4 +1,4 @@
-# lumen host configuration (~/.config/lumen/host.env) — consumed by lumen-host.service.
+# punktfunk host configuration (~/.config/punktfunk/host.env) — consumed by punktfunk-host.service.
 
 # Session / compositor environment (headless KWin example).
 XDG_RUNTIME_DIR=/run/user/1000
@@ -8,15 +8,15 @@ XDG_CURRENT_DESKTOP=KDE
 
 # Video source: `virtual` creates a per-client virtual output at the client's exact
 # resolution+refresh (the flagship mode); `portal` captures an existing monitor.
-LUMEN_VIDEO_SOURCE=virtual
+PUNKTFUNK_VIDEO_SOURCE=virtual
 
 # GPU zero-copy capture (EGL/Vulkan → CUDA → NVENC). Falls back to CPU automatically.
-LUMEN_ZEROCOPY=1
+PUNKTFUNK_ZEROCOPY=1
 
 # Optional overrides (apps.json is the primary mechanism for per-app settings):
-#LUMEN_COMPOSITOR=kwin          # kwin | mutter | gamescope | wlroots
-#LUMEN_GAMESCOPE_APP=vkcube     # nested command for ad-hoc gamescope sessions
-#LUMEN_INPUT_BACKEND=libei      # wlr | libei | gamescope | uinput
-#LUMEN_FEC_PCT=20               # video FEC overhead percent
-#LUMEN_PERF=1                   # per-stage timing logs
+#PUNKTFUNK_COMPOSITOR=kwin          # kwin | mutter | gamescope | wlroots
+#PUNKTFUNK_GAMESCOPE_APP=vkcube     # nested command for ad-hoc gamescope sessions
+#PUNKTFUNK_INPUT_BACKEND=libei      # wlr | libei | gamescope | uinput
+#PUNKTFUNK_FEC_PCT=20               # video FEC overhead percent
+#PUNKTFUNK_PERF=1                   # per-stage timing logs
 #RUST_LOG=info