feat: record at the source's size, only once capture works

Encoders and the segment output are created when the capture source first
has a picture, at its own size (portrait monitors record portrait) unless a
resolution is configured, so nothing is written while waiting for a source.
SPEC.md records both decisions; validation docs cover how sessions should end
when the game closes.

docs/validation.md

@@ -149,18 +149,28 @@ Windows status:
   its last frame after the stream stops, and only logs the stream state. The
   engine reads that log line (stable for the pinned OBS version). A more robust
   follow-up is watching the PipeWire node state directly.
-- **Closed windows on Wayland.** With Hyprland's portal, closing a captured
-  window leaves the stream running on its last frame, with no state change or
-  error. Capture cannot tell this apart from a static window, so sessions must
-  end from game process exit (SPEC: "Stop/finalise the session when its game
-  exits"), which the Steam/process detection milestone provides.
-- **Writing while waiting for capture.** The output starts before the source
-  has a picture, so a source that never arrives produces black segments. The
-  session timeline should show that time as missing footage, or the output
-  should start only once capture is usable.
-- **Canvas orientation.** The output canvas is fixed by the configured
-  resolution, so portrait or other-aspect sources are letterboxed. Decide
-  whether the canvas should follow the captured source.
+- **Ending a session when the game closes.** With Hyprland's portal, closing a
+  captured window leaves the stream running on its last frame, with no state
+  change or error, so capture cannot tell it from a static window. Monitor
+  capture never sees the game close at all. The session therefore ends from
+  the game's process exiting (SPEC: "Stop/finalise the session when its game
+  exits"); capture loss stays a failure signal, not the normal end:
+  - Linux: Steam runs each game under `reaper SteamLaunch AppId=<id>`, which
+    lives exactly as long as the game (Proton included). Watch that process
+    with a pidfd (`pidfd_open` + poll) rather than polling `/proc`.
+  - Windows: watch the game's process handle (`OpenProcess` +
+    `WaitForSingleObject`), found from Steam's `RunningAppID` registry value.
+    Game Capture reports size 0 when its game closes; the exit must be seen
+    first so a normal exit finalises the session instead of failing it.
+  - Hyprland's IPC does announce window closes, but it is compositor-specific
+    and cannot map a portal stream to a window, so it is not used.
+- **Nothing is written before capture works.** Encoders and the segment
+  output are created only once the source has a picture, so waiting in the
+  picker or for a stalled portal writes no footage.
+- **Recording size.** By default each recording is the captured window's or
+  monitor's own size, set when capture first has a picture (portrait monitors
+  record portrait). If the window is resized mid-session the size stays and
+  the picture is scaled to fit.
 - **Audio capture failures are not detected yet.** When the PulseAudio server
   was unreachable, recording continued with a silent track.
 - **Linux runtime dependencies.** The Linux OBS build links the system's