feat: export poses to COCO and MediaPipe keypoint formats (Fixes #22)

[laurentketterle-hub]

Changes

• Add src/poseguide/data/export_formats.py with COCO and MediaPipe round-trip conversion
• Add poseguide data export --format coco|mediapipe CLI command
• Add 9 round-trip tests preserving joint fidelity

Acceptance

• Round-trip tests on fixtures
• CLI export command (COCO + MediaPipe)

Fixes #22

[laurentketterle-hub]

/attempt 22

[laurentketterle-hub]

Friendly ping - PR open for 3 days. Anything needed to move this forward? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!

[laurentketterle-hub]

Hi maintainer! Just a friendly ping on this PR - it has been ready for review. Any feedback or updates needed? Thanks!