Add optional Nintendo 3DS MPO metadata parsing

[Copilot]

Nintendo 3DS cameras embed proprietary stereoscopic metadata in MPO files via APP2/NINT segments. This adds optional parsing to extract that data.

Changes

• NintendoMetadata struct: Stores raw bytes from APP2/NINT segments
• MPO.Nintendo field: Optional pointer field, nil when absent
• parseNintendoMetadata(): Scans for FF E2 markers with "NINT" identifier
• DecodeAll() enhancement: Buffers raw data to parse Nintendo segments after JPEG decode
• HasNintendoMetadata() helper: Convenience method for presence check

Usage

mpo, _ := mpo.DecodeAll(reader)

if mpo.HasNintendoMetadata() {
    // Access raw Nintendo stereoscopic metadata
    data := mpo.Nintendo.Raw
}

Fully backward compatible—existing code unchanged. Reference: https://3dbrew.org/wiki/MPO

Original prompt

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This section details on the original issue you should resolve

<issue_title>Potentially Optionally Parse Nintendo Specific MPO Headers</issue_title>
<issue_description>https://3dbrew.org/wiki/MPO has details on the implementation, it'll probably be worth poking eventually.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Potentially Optionally Parse Nintendo Specific MPO Headers #1

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