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.

crates/clipbin-obs/src/lib.rs

@@ -13,6 +13,6 @@ mod thread;
 
 pub use recorder::{
     Engine, EngineError, Health, PipeWireState, RecorderEvent, Recording, RecordingConfig,
-    SEGMENT_EXTENSION, StatusTracker, StopReason, VideoFormat,
+    SEGMENT_EXTENSION, StatusTracker, StopReason,
 };
 pub use runtime::{RuntimeError, RuntimeLayout};