feat: detect lost sources, stalled writes and low disk space

PipeWire capture keeps its size and last frame after its stream stops, so the
recorder follows the stream state OBS logs. Recording fails when the segment
on disk stops growing (OBS kept counting bytes into a dead muxer and missed a
full disk) and stops while enough space remains to finalise the last segment.
Failures stay visible after the output stops.

Adds a check command that verifies keyframe-aligned segments and a remux
across segment boundaries, and gives the rig desktop audio.

crates/clipbin-obs/Cargo.toml

@@ -10,6 +10,7 @@ publish.workspace = true
 
 [dependencies]
 clipbin-core.workspace = true
+fs4 = { version = "1.1", default-features = false, features = ["sync"] }
 libobs.workspace = true
 
 # Raw libobs calls are confined to src/ffi.rs, which opts back in to unsafe code.