VC3D: cast seeding rays on active plane

[LubuSeb]

Summary

Fixes #372 by making point-mode seeding rays follow the segmentation plane that provided the focus point:

• XY focus normals keep the existing XY ray behavior.
• XZ focus normals cast and preview rays across X/Z while holding Y constant.
• YZ focus normals cast and preview rays across Y/Z while holding X constant.
• The distance-transform sample plane and lookup coordinates now match the active ray plane.

Draw-mode path analysis is left unchanged.

Reproduction

The current point-mode seeding path always offsets X/Y and keeps Z constant when previewing or casting rays. That means a focus point set from the XZ or YZ segmentation panes still casts on the XY plane, even though issue #372 asks those panes to cast on XZ and YZ respectively.

Scope

Minimality/scope rationale: this patch is focused and only changes SeedingWidget.cpp; it does not include unrelated refactors. It adds small plane-mapping helpers and uses them in the point-mode preview/cast paths and the point-mode parameter preview. It does not change neural trace behavior, draw-mode path peak finding, segmentation growth, or any saved surface geometry.

Issue / Artifact Binding

Issue binding: #372 (challenge-prize:ScrollPrize/villa#372).

Artifact link: this pull request is the patch artifact for that issue. The Vesuvius Open Prizes page says VC3D improvements may be considered for Progress Prizes, but this PR is submitted as a normal issue fix and does not request or guarantee a payout, bounty, or reward.

Validation

Ran in Docker with ghcr.io/scrollprize/villa/volume-cartographer:builder-ubuntu-26.04:

cmake --build --preset ci-tests-gcc --target VC3D
cmake --build --preset ci-tests-gcc
ctest --preset ci-tests-gcc --output-on-failure

Result: 97/97 tests passed.

Also ran a Qt offscreen GUI startup smoke test because the builder image does not include xvfb-run or Xvfb:

QT_QPA_PLATFORM=offscreen timeout 8s build/ci-tests-gcc/bin/VC3D
GUI_SMOKE_RC=124

Exit code 124 is expected from timeout; VC3D started and initialized before the timeout stopped it.

Known Limits

I did not run an interactive desktop click-through. This PR is linked to issue #372 and may be relevant to the Vesuvius Progress Prize wishlist for VC3D improvements, but it does not claim or guarantee any prize award.

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