feat(capture): give CaptureFactory a proper error channel

Replace the nullable unique_ptr returned by CaptureFactory::create()
with CaptureResult<std::unique_ptr<CaptureSession>> using the new
CaptureError/CaptureResult pattern, mirroring codec/error.h. The stub
now reports 'not implemented yet' as an error instead of returning
nullptr. Update handoff memory with the review outcome and
forward-looking notes for phases 3, 5, and 7.

.agents/MEMORY.md

@@ -1,19 +1,31 @@
 # Project Memory — screen_cast
 
-Last updated: initial scaffold.
+Last updated: Phase 2 codec review fixes applied.
 
 ## Project state
 
-- 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.
+- Phase 2 codec implementation reviewed with valgrind; all confirmed defects
+  fixed on top of the capture-stub commit:
+  - decoder leaked every packet payload (`av_malloc` + direct `packet->data`
+    assignment bypassed the packet's owning `AVBufferRef`); payloads are now
+    allocated with `av_new_packet`.
+  - decoder extradata lacked `AV_INPUT_BUFFER_PADDING_SIZE`; FFmpeg's
+    extradata parser over-read the buffer (valgrind invalid reads). Now
+    allocated padded and zeroed.
+  - oversized encoded frames are rejected before the int cast.
+  - EAGAIN-retry loops in encoder/decoder now handle unexpected EOF instead
+    of retrying forever.
+  - fixed "RTP packet" → "AVPacket" error message in the encoder.
+- `CaptureFactory::create()` now returns
+  `CaptureResult<std::unique_ptr<CaptureSession>>` (variant with
+  `CaptureError`) instead of a nullable unique_ptr; the stub reports
+  "PipeWire capture is not implemented yet" as an error. New pattern lives in
+  `include/screencast/capture/error.h`, mirroring `codec/error.h`.
+- Validation: `meson test` 2/2 OK; valgrind on `test_codec_roundtrip` is now
+  clean (0 definite losses, 0 invalid reads); clang-format clean. See
+  `docs/RUNBOOK.md` for the reusable checks.
+- Phase 3 capture stub remains in place; remaining implementation: capture,
+  transport, rendering, and CLI/pipeline glue.
 
 ## Decisions
 
@@ -28,6 +40,8 @@ Last updated: initial scaffold.
 - Discovery: mDNS/Avahi.
 - Rendering: SDL2 or SDL3 + OpenGL.
 - Namespace: `sc`.
+- Module error results use per-module `std::variant<T, XError>` types
+  (`CodecResult`, `CaptureResult`) since C++20 has no `std::expected`.
 
 ## Active blockers
 
@@ -38,3 +52,17 @@ None.
 - GUI framework (Qt6 vs. none / CLI only) — deferred to later phase.
 - Hardware acceleration strategy (VAAPI / Vulkan Video / NVENC) — evaluate after
   software encode path works.
+
+## Forward-looking review notes (for later phases)
+
+- Encoder rejects non-packed strides in `make_input_frame`; PipeWire/portal
+  frames usually have alignment-padded strides — Phase 3 must pass the real
+  stride through to swscale instead of rejecting it.
+- `AV_CODEC_FLAG_GLOBAL_HEADER` suppresses in-band SPS/PPS; a receiver cannot
+  join mid-stream or recover after PLI without parameter sets. Phase 5 must
+  prepend SPS/PPS to keyframes or negotiate them in signaling.
+- Encoder sets no VBV (`maxrate`/`buffer_size`) — ABR only; add for smoother
+  UDP streaming in Phase 7.
+- No negative-path tests yet (bad config, bad stride, undersized buffer).
+- `to_annex_b_h264` sniffs AVCC vs Annex-B by content; if an AVCC-emitting
+  encoder is ever added, prefer an explicit config flag over the heuristic.

include/screencast/capture/capture.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "screencast/capture/error.h"
+
 #include <cstdint>
 #include <memory>
 #include <optional>
@@ -52,7 +54,7 @@ class CaptureSession {
 // Factory for the PipeWire / xdg-desktop-portal capture backend.
 class CaptureFactory {
   public:
-    static std::unique_ptr<CaptureSession> create(CaptureTarget target);
+    static CaptureResult<std::unique_ptr<CaptureSession>> create(CaptureTarget target);
 };
 
 } // namespace sc

include/screencast/capture/error.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <variant>
+
+namespace sc {
+
+struct CaptureError {
+    std::string message;
+};
+
+// C++20 does not provide std::expected. Use a variant-based result type so
+// fallible capture operations do not rely on exceptions.
+template <typename T> using CaptureResult = std::variant<T, CaptureError>;
+
+template <typename T> constexpr bool is_capture_error(const CaptureResult<T>& result) noexcept {
+    return std::holds_alternative<CaptureError>(result);
+}
+
+template <typename T> T& capture_value(CaptureResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> const T& capture_value(const CaptureResult<T>& result) {
+    return std::get<T>(result);
+}
+
+template <typename T> CaptureError& capture_error(CaptureResult<T>& result) {
+    return std::get<CaptureError>(result);
+}
+
+template <typename T> const CaptureError& capture_error(const CaptureResult<T>& result) {
+    return std::get<CaptureError>(result);
+}
+
+} // namespace sc

src/capture/pipewire_capture.cpp

@@ -4,10 +4,10 @@
 
 namespace sc {
 
-std::unique_ptr<CaptureSession> CaptureFactory::create(CaptureTarget /*target*/) {
+CaptureResult<std::unique_ptr<CaptureSession>> CaptureFactory::create(CaptureTarget /*target*/) {
     // Phase 3 placeholder. The PipeWire / xdg-desktop-portal capture backend
-    // will be implemented here once the codec round-trip is solidified.
-    return nullptr;
+    // will be implemented here once the portal integration lands.
+    return CaptureError{"PipeWire capture is not implemented yet"};
 }
 
 } // namespace sc