feat(capture): add Phase 3 PipeWire capture stub and config string safety

Add a linkable CaptureFactory stub so the capture API can be consumed

without unresolved symbols. Change remaining config string_view fields to

std::string to prevent dangling references. Update handoff memory.

.agents/MEMORY.md

@@ -4,10 +4,16 @@ Last updated: initial scaffold.
 
 ## Project state
 
-- Phase 1 complete: Meson build, C++20 module headers, agent workflow, clock
-  utility, and a passing unit test.
-- Active phase is now Phase 2 (software H.264 encode/decode).
-- No implementation yet for capture/codec/network/render.
+- Phase 2 follow-up review items addressed: encoder rate control is now
+  bitrate-only (removed CRF), encoder/decoder cache their SwsContext, configs
+  own their strings, bitstream helper is internal to the encoder with a NAL
+  length guard, FFmpeg open errors are reported, and the round-trip test
+  verifies Annex-B prefix and keyframes.
+- `.clang-format` added at repo root and clang-format enforcement added to
+  `AGENTS.md` and `cpp-meson-build/SKILL.md`.
+- Phase 3 capture now has a linkable stub (`src/capture/pipewire_capture.cpp`)
+  so the API can be consumed without an unresolved symbol.
+- Remaining implementation: capture, transport, rendering, and CLI/pipeline glue.
 
 ## Decisions
 
@@ -16,6 +22,8 @@ Last updated: initial scaffold.
 - Build system: Meson.
 - Capture: PipeWire + xdg-desktop-portal.
 - Encode/Decode: FFmpeg (libavcodec, libavutil, libswscale).
+- H.264 encoder path: software `libx264`, low-latency settings, Annex-B output.
+- Decoder receives SPS/PPS through `DecoderConfig.extradata`.
 - Transport: RTP over UDP; signaling via WebSocket/JSON.
 - Discovery: mDNS/Avahi.
 - Rendering: SDL2 or SDL3 + OpenGL.