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/src/error.rs

@@ -0,0 +1,56 @@
+//! Error type and the stable C ABI status codes it maps to.
+
+use thiserror::Error;
+
+/// The core's internal error type. Crosses the C ABI as a [`LumenStatus`] code.
+#[derive(Debug, Error)]
+pub enum LumenError {
+    #[error("invalid argument: {0}")]
+    InvalidArg(&'static str),
+    #[error("fec error: {0}")]
+    Fec(#[from] crate::fec::FecError),
+    #[error("crypto seal/open failed")]
+    Crypto,
+    #[error("malformed packet")]
+    BadPacket,
+    #[error("no complete frame available yet")]
+    NoFrame,
+    #[error("unsupported: {0}")]
+    Unsupported(&'static str),
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+}
+
+pub type Result<T> = core::result::Result<T, LumenError>;
+
+/// Stable C ABI status codes. `Ok` is 0; all errors are negative so callers can
+/// test `rc < 0`. Do not renumber existing variants — only append.
+#[repr(i32)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum LumenStatus {
+    Ok = 0,
+    InvalidArg = -1,
+    Fec = -2,
+    Crypto = -3,
+    BadPacket = -4,
+    NoFrame = -5,
+    Unsupported = -6,
+    Io = -7,
+    NullPointer = -8,
+    Panic = -99,
+}
+
+impl LumenError {
+    /// Map to the C ABI status code.
+    pub fn status(&self) -> LumenStatus {
+        match self {
+            LumenError::InvalidArg(_) => LumenStatus::InvalidArg,
+            LumenError::Fec(_) => LumenStatus::Fec,
+            LumenError::Crypto => LumenStatus::Crypto,
+            LumenError::BadPacket => LumenStatus::BadPacket,
+            LumenError::NoFrame => LumenStatus::NoFrame,
+            LumenError::Unsupported(_) => LumenStatus::Unsupported,
+            LumenError::Io(_) => LumenStatus::Io,
+        }
+    }
+}