Add feature maps (fmaps) mode for sliding-kernel feature extraction

[walkerbdev]

Feature Maps (fmaps) — Sliding Kernel Feature Extraction

• Slides a K×K (or K×K×K) kernel across each parent ROI, computes features on each patch, and returns spatial feature maps
• 2D workflow: generateChildRois() → processIntSegImagePair_fmaps() → processDataset_2D_fmaps() in new workflow_2d_fmaps.cpp
• 3D workflow: same pattern in new workflow_3d_fmaps.cpp
• In-memory (Python) path: processIntSegImagePairInMemory_fmaps() added to workflow_pythonapi.cpp
• CLI dispatches to fmaps workflows when --fmaps is set via main_nyxus.cpp
• EnvRoiSwapGuard RAII guard swaps env ROI data to child data during feature computation, restores on scope exit
• FmapArrayResult struct holds spatial map dimensions, origin, and flat feature data array per parent ROI
• Environment gains fmaps_mode, fmaps_kernel_radius, fmaps_kernel_size(), fmaps_prevents_arrow()

Output

• save_features_2_fmap_arrays() — builds spatial feature maps as flat arrays stored in ResultsCache (Python/buffer path); handles both 2D and 3D via parent_zmin/parent_d parameters
• save_features_2_csv_fmaps() — writes *_fmaps.csv with extra columns: parent_roi_label, kernel_center_x, kernel_center_y (CLI path)
• Refactored shared helpers (get_feature_column_names, collect_feature_values, write_feature_values_csv, write_feature_header_buffer, write_feature_values_buffer) replace ~1000 lines of duplicated feature-dispatch code across all existing output paths

Python API & Bindings

• Nyxus(features=[...], fmaps=True, fmaps_radius=2) — new constructor params with validation
• featurize(), featurize_directory(), featurize_files() return list-of-dicts in fmaps mode, each dict containing metadata + features dict mapping feature names to numpy arrays shaped (map_h, map_w) or (map_d, map_h, map_w)
• fmap_results_to_python() in C++ bindings converts FmapArrayResult → Python dicts with numpy arrays
• set_fmaps_imp() binding for runtime fmaps mode toggle
• New fmap_io.py: save_fmaps_to_tiff() and save_fmaps_to_nifti() export utilities

Tests

• New test_fmaps.h — 2D fmaps unit tests
• New test_fmaps_3d.h — 3D fmaps unit tests

Docs

• README, Classes.rst, cmdline_and_examples.rst updated with fmaps usage, Python API, and CLI options