Recording (Windows): don't copy the webcam frame when it has not changed #934

Part of #920 (rank 12). Effort S, risk low.

Problem

• The video writer asks the webcam for its latest frame on every screen tick (60/s) and compares the sequence number afterwards (main.cpp, webcam block of writeVideoFrames).
• WebcamCapture::copyLatestFrame copies the whole frame before that comparison (webcam_capture.cpp), and so does the DirectShow path.
• A 30 fps camera therefore costs about 30 useless full-frame copies a second: ~190 MB/s at 1080p NV12, ~750 MB/s at 4K, on the thread that paces the screen.

Proposal

• Pass the last seen sequence to copyLatestFrame and return early, without copying, when it has not changed (both capture backends).

Verify

• A unit test on the snapshot rule: an unchanged sequence copies nothing.
• On a real take: storeMs / readSampleMs in INFO: Webcam capture loop ended, and the helper's CPU, before and after.