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-host/src/inject.rs

@@ -0,0 +1,30 @@
+//! Input injection (plan §4): turn client [`lumen_core::input::InputEvent`]s into host
+//! input. Wayland-native via libei (`reis`) first; uinput as the universal fallback.
+
+use anyhow::Result;
+use lumen_core::input::InputEvent;
+
+/// Injects input events into the host session.
+pub trait InputInjector: Send {
+    fn inject(&mut self, event: &InputEvent) -> Result<()>;
+}
+
+/// Preferred injection backend.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Backend {
+    /// libei via `reis` — Wayland-native (RemoteDesktop portal).
+    Libei,
+    /// `/dev/uinput` — universal fallback, always available.
+    Uinput,
+}
+
+pub fn open(_backend: Backend) -> Result<Box<dyn InputInjector>> {
+    #[cfg(target_os = "linux")]
+    {
+        anyhow::bail!("libei/uinput injection not yet implemented (M2)")
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        anyhow::bail!("input injection requires Linux (libei/uinput)")
+    }
+}