feat: M3 — lumen/1 native streaming: real video at client mode + input over QUIC datagrams

The native protocol now does the real thing, end to end:

- Hello carries the client's requested mode; the host creates a NATIVE virtual output at
  exactly that size/refresh (same vdisplay backends as the GameStream path) and streams
  NVENC HEVC through the M1 Session (GF(2^16) Leopard FEC + AES-GCM, QUIC-negotiated).
- Input rides QUIC DATAGRAMS — encrypted, congestion-managed, no ENet retransmission
  spikes — decoded into lumen_core InputEvents and fed to the session's input injector.
- Frames are stamped with the capture wall clock; the reference client computes per-frame
  capture→reassembled latency percentiles and writes a playable .h265.
- m3-host gains --source synthetic|virtual + --seconds; the client gains --mode WxHxFPS,
  --out, --input-test (scripted mouse/keyboard datagrams).

VALIDATED live (gamescope session, xev nested): client requested 1280x720@120 → host
created gamescope at that mode → 1680/1680 frames over 14s, zero loss, valid HEVC;
pipeline latency p50 0.83ms / p95 1.2ms / p99 1.3ms (capture→encode→FEC→crypto→UDP→
reassembled, same-host clock); 176 input datagrams sent → injector (GamescopeEi) → 164
X events observed inside the nested session.

Known follow-on: slice-level sub-frame pipelining needs the NVENC SDK directly (libavcodec
emits whole AUs only) — the next big latency lever.

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

crates/lumen-host/src/main.rs

@@ -60,21 +60,24 @@ fn real_main() -> Result<()> {
         Some("zerocopy-probe") => zerocopy::probe(),
         // M0 pipeline spike.
         Some("m0") => m0::run(parse_m0(&args[1..])?),
-        // M3 seed: native lumen/1 host (QUIC control plane + UDP data plane).
+        // M3: native lumen/1 host (QUIC control plane + UDP data plane).
         Some("m3-host") => {
-            let port = args
-                .iter()
-                .skip_while(|a| *a != "--port")
-                .nth(1)
-                .and_then(|s| s.parse().ok())
-                .unwrap_or(9777);
-            let frames = args
-                .iter()
-                .skip_while(|a| *a != "--frames")
-                .nth(1)
-                .and_then(|s| s.parse().ok())
-                .unwrap_or(300);
-            m3::run(port, frames)
+            let get = |flag: &str| {
+                args.iter()
+                    .skip_while(|a| *a != flag)
+                    .nth(1)
+                    .map(String::as_str)
+            };
+            let source = match get("--source") {
+                Some("virtual") => m3::M3Source::Virtual,
+                _ => m3::M3Source::Synthetic,
+            };
+            m3::run(m3::M3Options {
+                port: get("--port").and_then(|s| s.parse().ok()).unwrap_or(9777),
+                source,
+                seconds: get("--seconds").and_then(|s| s.parse().ok()).unwrap_or(30),
+                frames: get("--frames").and_then(|s| s.parse().ok()).unwrap_or(300),
+            })
         }
         Some("-h") | Some("--help") | Some("help") | None => {
             print_usage();