Add MjpegFrameSource for live MJPEG streams

[Konstantysz]

Summary

• Implements MjpegFrameSource — a FrameSource (not FrameSeeker) backed by cv::VideoCapture with ffmpeg backend
• Configurable reconnect logic via CameraConfig (uri, reconnectDelayMs, maxReconnectAttempts, grabTimeoutMs)
• 8 TDD unit tests covering open/fail, grab, exhaustion, close, describe, factory, and FrameSeeker exclusion

Test plan

• OpenFailsOnUnreachableUri — returns false for unreachable HTTP URI
• OpenSucceedsOnLocalFileUri — opens local MJPEG .avi fixture
• GrabReturnsFrameWithCorrectMetadata — sequenceNumber increments, captureTime set
• GrabReturnsNulloptWhenExhausted — nullopt after frames run out
• CloseReleasesCapture — IsOpen false, Grab returns nullopt after Close
• DescribeContainsUri — Describe includes class name and URI
• FactoryReturnsNonNullPtr — factory returns non-null unique_ptr
• DoesNotImplementFrameSeeker — dynamic_cast to FrameSeeker returns nullptr
• All 169 tests pass (no regressions)

Closes #3, #4