fix(preview): preserve half-frame dimensions and IR defects

[hsnilsson]

AI-assisted comment (model: GPT-5).

Summary

• Report a half frame's cropped and split full-resolution dimensions even when its preview uses a reduced RAW decode.
• Use one pixel-bound calculation for both half-frame slicing and dimension reporting.
• Preserve thin IR defects when segmented RGBI previews are reduced across strip boundaries.
• Document half-frame Original resolution and the corresponding preview and IR invariants.

Relation to #1104, #1107, #1119, and #1121

This is the fourth and final focused extraction from #1104. As noted in the split comment, the larger draft is being divided into smaller PRs before review.

This draft depends on #1121, which is stacked on #1119 and #1107. Until those PRs merge and this branch is rebased, GitHub includes the earlier changes in this comparison.

Adjacent open work

#1112 changes half-frame auto-detection in the same service but does not change the slicing or dimension-reporting behavior fixed here.

#1108 changes calibrated RAW saturation in the preview manager but does not change half-frame geometry or segmented IR reduction.

Neither PR is included in this branch. A mechanical rebase may be needed if either merges first.

Verification

• Ruff formatting and lint passed.
• Ty type checking passed.
• 122 focused half-frame, preview-manager, and RAW-loader tests passed.
• Full suite: 5,722 passed, 40 skipped, and 14 deselected.