Add fixed-calibration THIN_PRISM_FISHEYE support for Caspar BA

[DuVogel87]

Summary

Adds draft THIN_PRISM_FISHEYE support to the Caspar bundle-adjustment backend for narrow fixed-calibration rig BA cases.

Supported scope in this PR:

• THIN_PRISM_FISHEYE
• fixed intrinsics
• fixed sensor_from_rig
• optimized points3D
• fixed-pose / points-only BA through the standalone/default bundle adjuster path
• active rig/image pose + points BA with fixed calibration through the standalone/default bundle adjuster path
• f32 generated Caspar kernels, with f64 kernels generated as well

Unsupported THIN_PRISM combinations fail explicitly instead of silently skipping observations.

Important scope note: this draft does not claim incremental mapper / local BA support for THIN_PRISM_FISHEYE yet. The incremental mapper can exercise additional local-BA gauge and fixed-variable combinations that are outside the narrow generated factor set in this PR.

Motivation

Caspar currently supports only a limited set of camera models. This PR adds fixed-calibration THIN_PRISM_FISHEYE rig factors for calibrated fisheye rigs.

The first use case is points-only BA after reconstruction. The second use case is fixed-intrinsics pose+point BA, which is closer to full BA while still avoiding variable intrinsics and sensor-from-rig refinement.

Implementation notes

COLMAP parameter order:

fx, fy, cx, cy, k1, k2, p1, p2, k3, k4, sx1, sy1

Caspar split constant layout:

principal_point = [cx, cy]
focal_and_extra = [fx, fy, k1, k2, p1, p2, k3, k4, sx1, sy1]

Projection follows COLMAP THIN_PRISM_FISHEYE behavior:

1. normalize pinhole coordinates,
2. apply fisheye theta = atan(r) mapping,
3. apply radial/tangential/thin-prism distortion in fisheye coordinates uu,vv,
4. project to pixels.

The branch currently contains four logical commits:

1. adapter/generator/test wiring for fixed-pose points-only support,
2. generated fixed-pose kernels,
3. fixed-calibration active-pose + point support with generated kernels,
4. draft diagnostics: a Caspar BriefReport() and an explicit unsupported THIN_PRISM three-point gauge test.

Tests

f32:

cmake -S . -B build-minimal-pr-v3-cuda-ceres-tests -GNinja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCUDA_ENABLED=ON \
  -DCMAKE_CUDA_ARCHITECTURES=86 \
  -DCASPAR_ENABLED=ON \
  -DCASPAR_USE_DOUBLE=OFF \
  -DTESTS_ENABLED=ON \
  -DGUI_ENABLED=OFF \
  -DCeres_DIR=/home/ubu/dev/src/ceres-cuda/install/lib/cmake/Ceres
cmake --build build-minimal-pr-v3-cuda-ceres-tests --target bundle_adjustment_caspar_test -j 10
./build-minimal-pr-v3-cuda-ceres-tests/src/colmap/estimators/bundle_adjustment_caspar_test

Result: 22/22 passed.

f64 THIN_PRISM-specific tests:

cmake -S . -B build-minimal-pr-v3-cuda-ceres-tests-f64 -GNinja \
  -DCMAKE_BUILD_TYPE=Release \
  -DCUDA_ENABLED=ON \
  -DCMAKE_CUDA_ARCHITECTURES=86 \
  -DCASPAR_ENABLED=ON \
  -DCASPAR_USE_DOUBLE=ON \
  -DTESTS_ENABLED=ON \
  -DGUI_ENABLED=OFF \
  -DCeres_DIR=/home/ubu/dev/src/ceres-cuda/install/lib/cmake/Ceres
cmake --build build-minimal-pr-v3-cuda-ceres-tests-f64 --target bundle_adjustment_caspar_test -j 10
./build-minimal-pr-v3-cuda-ceres-tests-f64/src/colmap/estimators/bundle_adjustment_caspar_test --gtest_filter='DefaultBundleAdjuster.ThinPrismFisheye*'

Result: 5/5 THIN_PRISM tests passed.

A previous full f64 local run still had two non-THIN merged-calibration failures in this environment, so this remains a draft for review rather than ready-to-merge.

Limitations

This PR intentionally does not add:

• incremental mapper / local BA support for THIN_PRISM_FISHEYE,
• variable THIN_PRISM intrinsics,
• refine_sensor_from_rig,
• active pose with fixed points only,
• pose tangent component masks,
• full Target-B gauge/mask research machinery,
• a full THIN_PRISM factor-family expansion.

This is a narrow fixed-calibration implementation, not a complete Caspar THIN_PRISM feature set.