feat(calibration): accept joined H.264 captures

[shark0F0497]

Summary

• add calibration-only preprocessing for joined H.264 captures without modifying jobs/stereo-split
• decode Annex-B H.264 with FFmpeg, preserve source IMU, and write calibration-ready stereo JPEG topics
• preserve the existing joined-JPEG path through the legacy splitter
• reject missing IMU, mixed codecs, and invalid multi-picture H.264 messages
• configure HTTPS Ubuntu mirrors for amd64 calibration image builds

Validation

• make calibration-test (15 tests)
• Python compileall and git diff --check
• full Go test suite with race and coverage
• calibration image build and container smoke checks
• end-to-end run on a 2.16 GB DECXIN capture: 5,248 stereo frames, 57,717 IMU samples, no skipped frames, calibration succeeded
• camera calibration max reprojection error: 0.307218 px (excellent)
• camera-IMU calibration max reprojection error: 0.911241 px (excellent)

The image validation used archebase_calib revision 686b727be4137452079e66f42f735e907be4fe92.