Re-enable mecka_to_zarr with required episode metadata #738

#736 disables egomimic/scripts/mecka_process/mecka_to_zarr.py (MeckaDatasetConverter and its CLI). It now raises NotImplementedError.

The converter predates the required episode metadata and mandatory intrinsics. To re-enable it:

• Pass task_name, operator and lab to ZarrWriter.create_and_write as real arguments. These can never be waived.
• Pass scene and objects as values, or as MissingMetadata() when the source has none.
• Pass intrinsics={"front_1": <3x4 K>}.
• Stop putting writer-owned keys (objects, intrinsics, lab, ...) in metadata_override; the writer now raises on them.
• Reconverted zarrs need the fixed left-wrist convention.

See INTERNAL_DATA.md §4.3. The mecka_mono_to_zarr converter on the #715/#709 line has the same problem.