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>

tools/latency-probe/src/main.rs

@@ -0,0 +1,11 @@
+//! `latency-probe` — glass-to-glass latency measurement (plan §10, M3).
+//!
+//! Renders a timestamp/QR on the host, reads it back on the client capture (or a
+//! photodiode for true photons), and tracks p50/p99. Build this before optimizing
+//! anything, so regressions are quantifiable.
+//!
+//! Status: scaffold.
+
+fn main() {
+    println!("latency-probe: scaffold (M3 — render timestamp/QR on host, read back on client; track p50/p99)");
+}