XMP: retain focal-plane rational forms

[swackhamer]

Summary

• preserve raw XMP focal-plane-resolution rational forms as metadata value forms
• use those forms for Canon sensor-diagonal composite calculation
• add a regression test for the derived 35 mm scale factor

Validation

• cargo test parsers::xmp::tests::xmp_sidecar_carries_unreduced_focal_plane_resolution_into_canon_sensor_diag --lib -- --exact
• pinned ExifTool 13.59 XMP comparison: 179/193 -> 184/193 instances; no regressions
• just pre-commit (format, header, clippy, table conformance, release all-features tests)