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-core/cbindgen.toml

@@ -0,0 +1,28 @@
+language = "C"
+pragma_once = true
+include_guard = "LUMEN_CORE_H"
+autogen_warning = "/* Generated by cbindgen from lumen-core. Do not edit by hand. */"
+header = "/* lumen-core C ABI — see crates/lumen-core/src/abi.rs */"
+style = "type"
+cpp_compat = true
+tab_width = 4
+documentation = true
+documentation_style = "c99"
+
+[parse]
+parse_deps = false
+
+[export.rename]
+"InputEvent" = "LumenInputEvent"
+"InputKind" = "LumenInputKind"
+
+# QualifiedScreamingSnakeCase already qualifies each variant with the enum name
+# (LumenStatus::Ok -> LUMEN_STATUS_OK); do NOT also set prefix_with_name or it doubles.
+[enum]
+rename_variants = "QualifiedScreamingSnakeCase"
+
+[fn]
+sort_by = "None"
+
+[struct]
+derive_eq = false