feat: M4 groundwork — lumen/1 client connector in the C ABI + SwiftUI client scaffold

The shared-core architecture pays off: platform clients now link ONE Rust library that
does the entire lumen/1 protocol, and only add decode/present/input on top.

lumen-core:
- client.rs (quic feature): NativeClient — QUIC handshake + UDP data plane + input
  datagrams on internal threads; embedder surface = connect / next_frame / send_input.
- abi.rs: lumen_connect / lumen_connection_next_au (borrow-until-next-call, matching
  lumen_client_poll_frame semantics) / lumen_connection_send_input / lumen_connection_mode /
  lumen_connection_close. Guarded in the generated header by LUMEN_FEATURE_QUIC (cbindgen
  [defines] mapping), so the checked-in header is stable across feature sets.
- error.rs: append-only LumenStatus additions Timeout (-9) and Closed (-10).
- TESTED end-to-end through the C ABI: in-process lumen/1 host, lumen_connect pulls 25
  byte-verified frames, sends input, closes (m3.rs::c_abi_connection_roundtrip).

Apple client (clients/apple — SCAFFOLD, written on Linux, first Xcode build pending):
- scripts/build-xcframework.sh: cargo per Apple target → universal staticlib + header
  (LUMEN_FEATURE_QUIC pre-defined) + modulemap → LumenCore.xcframework.
- Package.swift (LumenKit) + Swift sources: LumenConnection (ABI wrapper), AnnexB
  (in-band VPS/SPS/PPS → CMVideoFormatDescription, Annex-B → AVCC CMSampleBuffers with
  DisplayImmediately), StreamView (SwiftUI over AVSampleBufferDisplayLayer — stage-1
  presenter that hardware-decodes compressed HEVC itself), InputCapture (GCMouse raw
  deltas + GCKeyboard HID→VK).
- README.md is the full handoff for the next (Mac-side) agent: build steps, ABI contract,
  first-light test recipe against the Linux host, stage-2 (VT+Metal pacing) plan, and the
  known host-side gaps (single-session m3-host, no lumen/1 audio yet, gamepad kinds not
  yet routed in m3's injector, seed-stage trust).

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

crates/lumen-core/src/error.rs

@@ -19,6 +19,10 @@ pub enum LumenError {
     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, LumenError>;
@@ -37,6 +41,8 @@ pub enum LumenStatus {
     Unsupported = -6,
     Io = -7,
     NullPointer = -8,
+    Timeout = -9,
+    Closed = -10,
     Panic = -99,
 }
 
@@ -51,6 +57,8 @@ impl LumenError {
             LumenError::NoFrame => LumenStatus::NoFrame,
             LumenError::Unsupported(_) => LumenStatus::Unsupported,
             LumenError::Io(_) => LumenStatus::Io,
+            LumenError::Timeout => LumenStatus::Timeout,
+            LumenError::Closed => LumenStatus::Closed,
         }
     }
 }