fix(neighbors): implement closest-neighbor dist/ang and cap PERCENT_T…

[darkclad]

…OUCHING

Two production defects:

• CLOSEST_NEIGHBOR{1,2}DIST/ANG and ANG_BW_NEIGHBORS* need CENTROID_X/Y from Basic Morphology. When only neighbor features were requested, Basic Morphology was skipped in the trivial-ROI reduce, so the centroids stayed 0 and the closest-neighbor distances/angles came out 0. Run Basic Morphology whenever neighbor features are requested (reduce_trivial_rois.cpp).
• PERCENT_TOUCHING summed a per-neighbor touch count, so a contour pixel adjacent to several neighbors was counted repeatedly and the total could exceed 100% (e.g. 127%). Build a deduped per-ROI mask of contour pixels touching ANY neighbor (8-connected adjacency) and divide by contour length -> <= 100% (neighbors.cpp).

Refresh the one affected vetted golden in test_neighbors_2d.h (label 2 PERCENT_TOUCHING 33.33 -> 66.67; all closest-neighbor goldens unchanged) and add tests/python/test_feature_bugs.py::test_neighbor_distance_and_percent_touching.

[darkclad]

Justification — golden value change for main-neighbors-closest-and-percent-touching

Branch: main-neighbors-closest-and-percent-touching (split from main-feature-validation, based on main)
Source fix: src/nyx/features/neighbors.cpp, src/nyx/reduce_trivial_rois.cpp
Test file touched: tests/test_neighbors_2d.h (1 golden: label 2 PERCENT_TOUCHING)

The bugs

• fix variable type to address windows build issue #13 PERCENT_TOUCHING could exceed 100% (neighbors.cpp). The old code added a per-neighbor
  touch count, so a contour pixel adjacent to several neighbours was counted once per neighbour and
  the total could exceed the contour length (e.g. 127%). Fix: build a deduped per-ROI mask of
  contour pixels touching ANY neighbour (8-connected adjacency, sqdist ≤ 2) and divide the distinct
  touching-pixel count by the contour length → guaranteed ≤ 100%.
• Better cache friendliness #12 CLOSEST_NEIGHBOR{1,2}_DIST/ANG were 0 in production (reduce_trivial_rois.cpp). Those
  features need CENTROID_X/Y, which are produced by Basic Morphology. When a user requested only
  neighbour features, Basic Morphology was not run, so the centroids stayed 0 and the closest-neighbour
  distances/angles came out 0. Fix: run Basic Morphology whenever neighbour features are requested.

Golden change (1 key)

Label	Golden	Old	New	Why
2	PERCENT_TOUCHING	33.333…	66.666…	The deduped, adjacency-based touching fraction for this fixture ROI.

Only this one value changed. Every other neighbour golden (NUM_NEIGHBORS, all
CLOSEST_NEIGHBOR*_DIST/ANG, ANG_BW_NEIGHBORS_*, and the other labels' PERCENT_TOUCHING) is
unchanged — the C++ unit test calls Basic Morphology explicitly before the neighbour reduce, so
its centroids are always populated and bug #12 (a production-only reduce-ordering defect) does not
manifest in the C++ fixture. That the whole suite passes with only label 2's PERCENT_TOUCHING
updated confirms the closest-neighbour reimplementation is value-preserving on this fixture.

Why this is not fudging

• Bounded invariant: the fix's purpose is PERCENT_TOUCHING <= 100%; label 2 stays well within
  bounds. The old summing scheme is the one that produced impossible >100% values (exercised by the
  Python test on touching squares).
• Minimal blast radius: exactly one golden moved; all others are byte-identical to main.
• Value is byte-identical to the oracle-validated main-feature-validation (copied, not invented);
  only the shared two-table→one-table refactor was dropped.

Regression guards (Python, production path)

tests/python/test_feature_bugs.py::test_neighbor_distance_and_percent_touching on two 4×4 squares
2 px apart asserts, through the production featurize() path:

• CLOSEST_NEIGHBOR1_DIST > 0 for all ROIs — catches bug Better cache friendliness #12 (was 0 when centroids weren't computed).
• PERCENT_TOUCHING <= 100 and == 0 (a 2 px gap → neighbours but not touching) — catches bug fix variable type to address windows build issue #13.

CI

Full suite green on this branch: C++ runAllTests 683/683; pytest tests/python/ 49 passed, 1 skipped.