Scaffold C++20 screencast project with Meson, agent workflow, and phase plan

docs/PHASES.md

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+# Development Phases
+
+This file breaks the project into incremental milestones. Each phase produces a
+working, testable slice of functionality. Do not start a phase until the
+previous one is validated.
+
+## Phase 1 — Project Skeleton
+
+**Goal**: configure, compile, and run tests with no real dependencies.
+
+- [x] Meson build files (`meson.build`, `meson_options.txt`).
+- [x] Public module headers with `namespace sc`.
+- [ ] Unit test that exercises a trivial utility function.
+- [ ] `README.md` with build instructions.
+
+**Validation**: `meson setup build && meson compile -C build && meson test -C build`.
+
+## Phase 2 — Software H.264 Encode / Decode
+
+**Goal**: encode raw pixel buffers to H.264 and decode them back, purely with
+FFmpeg software paths.
+
+- Add `libavcodec`, `libavutil`, `libswscale` dependencies.
+- Implement `EncoderFactory::create()` and `Encoder::encode()`.
+- Implement `DecoderFactory::create()` and `Decoder::decode()`.
+- Round-trip test: synthetic RGB frames → H.264 → decoded RGB.
+
+**Validation**: unit test produces visually/structurally correct round-trip
+frames.
+
+## Phase 3 — PipeWire Screen Capture
+
+**Goal**: capture the desktop and feed frames into the encoder.
+
+- Add `libpipewire-0.3` dependency.
+- Implement `CaptureFactory::create()` using the xdg-desktop-portal.
+- Wire `CaptureSession::next_frame()` → `Encoder::encode()` in a local smoke
+  test that just writes a few encoded frames to disk.
+
+**Validation**: manual run on a real Linux desktop session produces a valid
+H.264 bitstream.
+
+## Phase 4 — RTP Framing
+
+**Goal**: packetize NAL units into RTP and depacketize them.
+
+- Implement `RtpHeader` and `RtpPacket` serialize/parse.
+- Add H.264 NAL splitting and FU-A fragmentation.
+- Unit test for serialization, fragmentation, and reassembly.
+
+**Validation**: unit tests cover single-NAL and fragmented packet paths.
+
+## Phase 5 — Local UDP Sender → Receiver Loopback
+
+**Goal**: send RTP packets over UDP and render the result locally.
+
+- Implement `RtpTransport` with ASIO or raw UDP sockets.
+- Wire sender pipeline: capture → encode → RTP → localhost UDP.
+- Wire receiver pipeline: localhost UDP → RTP → decode → renderer.
+- Add SDL2/3 dependency and a minimal `Renderer`.
+
+**Validation**: `screencast --send` and `screencast --receive` on the same
+machine show the captured desktop in a window.
+
+## Phase 6 — LAN Signaling and Discovery
+
+**Goal**: two peers on the same LAN can find each other and negotiate a
+session.
+
+- Implement mDNS/DNS-SD discovery with Avahi.
+- Implement JSON WebSocket signaling.
+- Extend CLI with `--peer-address` or `--discover`.
+
+**Validation**: two machines on the same LAN connect without hard-coded IP
+addresses.
+
+## Phase 7 — Resilience and Polish
+
+**Goal**: loss recovery, hardware acceleration, and packaging.
+
+- NACK / PLI feedback loop.
+- Jitter buffer on the receiver.
+- VAAPI/NVENC hardware encode probes and fallback.
+- Optional GUI target behind `meson -Dgui=true`.
+- `.desktop` file, icon, packaging notes.
+
+**Validation**: sustained streaming under packet loss; hardware accel smoke
+where available.
+
+## Current phase
+
+Phase 1 — skeleton and tooling.