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>

crates/punktfunk-core/src/error.rs

@@ -0,0 +1,64 @@
+//! Error type and the stable C ABI status codes it maps to.
+
+use thiserror::Error;
+
+/// The core's internal error type. Crosses the C ABI as a [`PunktfunkStatus`] code.
+#[derive(Debug, Error)]
+pub enum PunktfunkError {
+    #[error("invalid argument: {0}")]
+    InvalidArg(&'static str),
+    #[error("fec error: {0}")]
+    Fec(#[from] crate::fec::FecError),
+    #[error("crypto seal/open failed")]
+    Crypto,
+    #[error("malformed packet")]
+    BadPacket,
+    #[error("no complete frame available yet")]
+    NoFrame,
+    #[error("unsupported: {0}")]
+    Unsupported(&'static str),
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("timed out")]
+    Timeout,
+    #[error("session closed")]
+    Closed,
+}
+
+pub type Result<T> = core::result::Result<T, PunktfunkError>;
+
+/// Stable C ABI status codes. `Ok` is 0; all errors are negative so callers can
+/// test `rc < 0`. Do not renumber existing variants — only append.
+#[repr(i32)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PunktfunkStatus {
+    Ok = 0,
+    InvalidArg = -1,
+    Fec = -2,
+    Crypto = -3,
+    BadPacket = -4,
+    NoFrame = -5,
+    Unsupported = -6,
+    Io = -7,
+    NullPointer = -8,
+    Timeout = -9,
+    Closed = -10,
+    Panic = -99,
+}
+
+impl PunktfunkError {
+    /// Map to the C ABI status code.
+    pub fn status(&self) -> PunktfunkStatus {
+        match self {
+            PunktfunkError::InvalidArg(_) => PunktfunkStatus::InvalidArg,
+            PunktfunkError::Fec(_) => PunktfunkStatus::Fec,
+            PunktfunkError::Crypto => PunktfunkStatus::Crypto,
+            PunktfunkError::BadPacket => PunktfunkStatus::BadPacket,
+            PunktfunkError::NoFrame => PunktfunkStatus::NoFrame,
+            PunktfunkError::Unsupported(_) => PunktfunkStatus::Unsupported,
+            PunktfunkError::Io(_) => PunktfunkStatus::Io,
+            PunktfunkError::Timeout => PunktfunkStatus::Timeout,
+            PunktfunkError::Closed => PunktfunkStatus::Closed,
+        }
+    }
+}