test(morphology): vet DIAMETER_EQUAL_PERIMETER + ORIENTATION + EROSIONS_2_VANISH -> 80/113

[darkclad]

Vet three more 2D morphology features against analytic / scikit-image oracles (morphology 77 -> 80), and document why four others in the "future-vettable" list are not cleanly promotable.

Promoted:

• DIAMETER_EQUAL_PERIMETER -> analytic. == PERIMETER/pi (contour.cpp), pure double arithmetic on the vetted PERIMETER; recomputed at 1e-9 in the existing documented-formula-conformance test (same treatment as CIRCULARITY/ROUNDNESS).
• ORIENTATION -> skimage. skimage regionprops measures the major-axis angle from the row axis; Nyxus measures the same ellipse from the x axis, so NYXUS == 90 - degrees(skimage.orientation) = 70.4174. Matches to 10 decimals because the ellipse angle is invariant to the pixel-size second-moment correction (it shifts mu20 and mu02 equally). New gtest test_shape2d_skimage_orientation_and_erosions + tests/vetting/oracles/gen_morphology_skimage.py.
• EROSIONS_2_VANISH -> skimage. Nyxus' 3x3 (8-connected) erosion count == skimage binary_erosion(square(3)) = 1; disk(1)/4-connected gives 2, so the test also pins the connectivity convention.

Documented (stay regression, with reasons in the registry notes):

• GEODETIC_LENGTH: geo_len_thickness.cpp truncates PERIMETER to size_t and uses integer division, so its value (10.0) deviates ~11% from the real-valued rectangle formula (~11.13). Vetting it analytically would bake in the truncation -- a latent precision issue to fix first.
• ROI_RADIUS_MEAN/MAX: mean/max per-pixel min SQUARED distance to the ROI's own contour pixels (roi_radius.cpp); scipy distance_transform_edt uses a different reference (distance to background, not squared), so no standard tool reproduces it.
• EROSIONS_2_VANISH_COMPLEMENT: degenerate 0 on this fixture; needs a holed fixture.
• MAJOR/MINOR_AXIS_LENGTH, ECCENTRICITY: ~1.4% moment-normalization gap vs skimage, left regression.

gtest 717/717; pytest 72 passed (7 arrow fails are the tiff-only local build, not regressions). Coverage regenerated (604 -> 607 / 758).