Fix -Wunused-result warning in ThreadPool::AddTask - #4958

## Change
Integrate the upstream fix for an ignored return-value warning in task error
collection. The checker still calls shared_future::get() to propagate failures;
the explicit void cast discards only the returned value. Include the upstream
regression for an exception from a value-returning task.

## Upstream commits
- e6644e7 Fix -Wunused-result warning in ThreadPool::AddTask (colmap#4958)

## Validation
- Linux threading suite: 45/45 passed, no skips or disabled tests, including
  WaitPropagatesExceptionWithReturnValue.
- Incremental build: two threading translation units rebuilt; fresh test link
  against retained archives, with compiler, input and runtime-provider checks.

Totally vibe coded

Integrate the upstream fix for an ignored return-value warning in task error collection.

## Change

Keep calling `shared_future::get()` so task failures propagate, and explicitly discard only its returned value. The upstream regression submits a value-returning task that throws and checks that waiting reports the exception.

## Upstream commits

- e6644e7 Fix -Wunused-result warning in ThreadPool::AddTask (colmap#4958)

## Validation

The Linux threading suite passed 45 cases with no skips or disabled tests, including `WaitPropagatesExceptionWithReturnValue`. Two threading translation units were rebuilt and a fresh test link used retained archives with compiler, input and runtime-provider checks.

AI disclosure: Totally vibe coded

Refresh the experimental alternative with fork main while retaining its previous head as a parent. The resulting tree is exactly main plus the isolated shared-current-cost-cache deletion: one CUDA file, +1/-25.

## Change

Read the existing cost map directly during the ordered sampled-cost fold. Preserve current plane propagation, source packing, scheduling and fallback mathematics. The cache originates in custom a37c1c3, MVS: Reuse sampled current costs (GPU perf). The earlier alternative is retained through 8481f89; its original experiment was 8b1412f.

Historical Fountain11 photo+geometry at 3072 recorded 76.85 -> 78.82 seconds, one observation per arm, on the older revision. Retain this inconclusive/adverse observation with its earlier revision; current performance has not been measured. No performance experiment was reopened.

## Validation

- Exact candidate CUDA owner compiled with CUDA 13.2 for compute_86/sm_86. In a bounded retained-support context on RTX 3060, both matched targets passed all 13 selected cases: propagation, ordered source folding, checkpoint-HMM and paired-sweep preservation.
- Four complete inputs and four outputs matched across executables: 8,492,128 bytes per arm. Maps were finite with positive depth and checked ordered graph IDs. Complete witnesses use checkpoint_hmm=false; separate checkpoint tests cover both settings without full cross-executable true-mode witnesses.
- Direct owners, link maps and actual loaded providers were checked; managed jobs ended and released their bookings. Later host resize/warp changes have separate owner qualification. This is not a whole-branch build, reconstruction-quality, portability or current-performance result.

## Upstream commits carried forward

- e52aa8e Move pycolmap macOS CI from macos-14 to macos-15 runner (colmap#4933)
- 8007d1a Preserve float weight ordering in maximum spanning trees (colmap#4928)
- dd84102 Bump pypa/gh-action-pypi-publish from 1.14.2 to 1.14.3 in /.github/workflows (colmap#4942)
- 721e185 Add FlexGlue feature matcher (colmap#4908)
- d0a411f FlexGlue: cache the dense features on the GPU (colmap#4915)
- 105817b Hoist camera model dispatch per row in image warps (colmap#4935)
- 27ffad4 Make benchmark translation-angle errors invariant to reconstruction scale (colmap#4932)
- e8f3522 Resolve benchmark paths before changing subprocess directories (colmap#4931)
- f664d5d Clamp visibility coordinates before unsigned conversion (colmap#4943)
- 138119b Keep thin bitmap thumbnails nonempty (colmap#4927)
- b16ed5f Count rig frame covisibility once per 3D point (colmap#4893)
- 93dbbcb Add colmap-review agent skill for code review (colmap#4934)
- 2a6f695 Fix sift_test build on MSVC without the GUI (colmap#4955)
- 78cc729 Fix default prior_position_loss_scale to sqrt(chi2_95_3dof) (colmap#4936)
- 8653c6b Estimate two-view relative rotation covariance (colmap#4950)
- e1317dd FlexGlue: match keypoints in the resized descriptor input frame (colmap#4949)
- 3db6eb0 Extract ConvertPosePriorsToENU and skip pose priors without position (colmap#4937)
- 77cbe48 Store and estimate rotation covariances on PoseGraph (colmap#4951)
- 30a1a81 Support covariance whitening in CeresRotationAverager (colmap#4952)
- e6644e7 Fix -Wunused-result warning in ThreadPool::AddTask (colmap#4958)
- 5b6da88 Clear descriptor-index results for empty queries (colmap#4960)
- d1d92d9 Add COVARIANCE reweighting for Ceres rotation averaging (colmap#4953)
- 761d90d Enable covariance-weighted rotation averaging in global pipelines (colmap#4823)
- fdee2e1 Reuse the pose graph of GlobalPipeline in the component mappers (colmap#4912)

AI disclosure: Totally vibe coded