feat: Phase 1 MVP with audit fixes — Wayland screen capture + VAAPI encoding

Phase 1 MVP implementation of wl-webrtc: Wayland screen capture tool
with hardware-accelerated VAAPI H.264 encoding and WebTransport output.

Includes all 9 runtime bug fixes from code audit (fix-audit-issues plan):

CRITICAL:
- C2: h264_metadata BSF with repeat_sps/repeat_pps in encode pipeline
- C4: FpsLimit wired as timing gate in on_copy_complete

HIGH:
- C3+A2: DRM device discovery via dmabuf feedback MainDevice event,
  unified resolve_drm_path() helper (CLI > compositor > auto > fallback)
- H2: Separate physical_size (mm) from mode_size (pixels) in wl_output
- H1+A3: Multi-output warning + named-output-not-found error

MEDIUM:
- M5: tv_sec u32->u64 to avoid Y2106 timestamp truncation
- M4: Guard against SHM Buffer event (DMA-BUF only)

Key components:
- src/avhw.rs: FFmpeg VAAPI encoder + filter graph + BSF pipeline
- src/state.rs: Wayland event loop + output negotiation + screencopy
- src/cap_wlr_screencopy.rs: wlr-screencopy capture source
- src/fps_limit.rs: Frame rate limiting with configurable target
- src/transform.rs: Frame format conversion utilities

src/args.rs

@@ -0,0 +1,45 @@
+use clap::Parser;
+
+#[derive(Parser, Debug, Clone)]
+#[command(name = "wl-webrtc", about = "Wayland screen capture and encoding tool")]
+pub struct Args {
+    /// Output file path (e.g., output.mp4, output.mkv)
+    #[arg(short, long)]
+    pub output: String,
+
+    /// Wayland output name to capture
+    #[arg(long)]
+    pub output_name: Option<String>,
+
+    /// Target frames per second
+    #[arg(long, default_value_t = 30)]
+    pub fps: u32,
+
+    /// Video codec (h264 only for MVP)
+    #[arg(long, default_value = "h264")]
+    pub codec: String,
+
+    /// Hardware acceleration method (vaapi only for MVP)
+    #[arg(long, default_value = "vaapi")]
+    pub hw_accel: String,
+
+    /// DRM render device path (e.g., /dev/dri/renderD128)
+    #[arg(long)]
+    pub drm_device: Option<String>,
+
+    /// Target bitrate in bits per second
+    #[arg(long)]
+    pub bitrate: Option<u64>,
+
+    /// Group of Pictures (GOP) size
+    #[arg(long)]
+    pub gop_size: Option<u32>,
+
+    /// Enable verbose logging
+    #[arg(short, long)]
+    pub verbose: bool,
+
+    /// Port for WebTransport server (Phase 2, unused in MVP)
+    #[arg(long, default_value_t = 0)]
+    pub port: u16,
+}