feat: M1 lumen-core (FEC/crypto/packet/session + C ABI) and workspace scaffold

Ground-up low-latency streaming stack per docs/implementation-plan.md. M1 is
complete and tested; Linux host backends are cfg-gated stubs to be filled in on
real hardware (M0/M2).

lumen-core (built + tested on macOS/aarch64 — 21 tests):
- fec: ErasureCoder over GF(2^8) (reed-solomon-erasure, Moonlight-compatible)
  and GF(2^16) Leopard-RS (reed-solomon-simd, the >1 Gbps wall-breaker); proptested
- packet: zero-copy #[repr(C)] framing, multi-block, FEC-aware reassembly
- crypto: AES-128-GCM with per-direction nonce salts + sequence-as-AAD
- session: host submit / client poll hot paths + input; loopback & UDP transports
- abi: opaque handles, versioned LumenConfig, panic guards; cbindgen-generated header
- acceptance: Rust loopback+proptest and a C harness that links the staticlib

Scaffold (compiles green on all platforms): lumen-host (vdisplay/capture/encode/
inject/web/pipeline seams under cfg(linux)), lumen-client-rs, tools/{loss-harness,
latency-probe}, Apple/Android client stubs, Gitea CI, docs.

Hardened against a multi-agent adversarial review (13 verified findings fixed,
regression-tested): reassembler memory-DoS bounds + block-consistency validation,
GCM nonce-reuse direction separation, ABI struct_size guard + range checks, FEC
shard-length guards, shard_payload datagram bound, key zeroization + Debug redaction.

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

crates/lumen-client-rs/src/main.rs

@@ -0,0 +1,22 @@
+//! `lumen-client-rs` — the reference client (plan M4). Exists to exercise the `lumen/1`
+//! (P2) transport: `lumen_core` pulls reassembled, FEC-recovered access units; decode via
+//! VAAPI; present via wgpu/Vulkan aligned to client vsync (frame pacing, plan §7).
+//!
+//! Status: scaffold. The client side of `lumen_core` ([`lumen_core::Session::poll_frame`])
+//! is already complete and tested; this binary wires it to a real decoder + presenter.
+
+fn main() {
+    tracing_subscriber::fmt()
+        .with_env_filter(
+            tracing_subscriber::EnvFilter::try_from_default_env().unwrap_or_else(|_| "info".into()),
+        )
+        .init();
+
+    tracing::info!(
+        "lumen-client-rs scaffold (lumen_core ABI v{})",
+        lumen_core::ABI_VERSION
+    );
+    tracing::info!(
+        "intended flow: lumen_core::Session(client) over UDP → poll_frame → VAAPI decode → wgpu present"
+    );
+}