fix: reject incomplete NTv2 bicubic stencils at subgrid edges

[timothyanderson096-ocdealcheck]

Problem

Bicubic NTv2 interpolation assumes a complete 4x4 neighbourhood without checking that it stays inside the selected subgrid. Near the edges, this can wrap into another row, interpret header or neighbouring-subgrid bytes as nodes, or read beyond EOF. Issue #170 reports incorrect coordinate shifts and binary read failures.

Change

Validate row and column bounds in SubGrid.ntv2_bicubic before any file access. Incomplete stencils now raise an actionable ValueError directing callers to method='bilinear'. The requested interpolation method is not silently changed. Interior bicubic calculations, subgrid selection and existing north/west extent exclusions are preserved.

Document the boundary behaviour in the interpolation and transformation APIs and the NTv2 guide.

Validation

• Existing baseline: 74 unittest tests passed.
• Seven new regression tests use generated NTv2 binary fixtures: edge bands and corners in first/last subgrids, small grids, direct-method file access, nonlinear interior interpolation, extent semantics and the ntv2_2d wrapper.
• Before the fix, the new suite produced 19 failing and 8 erroring subtests.
• After the fix: all 81 unittest tests passed.
• Black applied to the new tests and changed Python regions; git diff --check passed.

Validation used synthetic grids. The reporter's real-grid examples and deployed government services were not tested. This fixes invalid stencil reads; it does not implement one-sided bicubic interpolation at boundaries.

Fixes #170

Prepared by Tim Anderson, Spotted Ridge Engineering / OC Labs, Australia, with OpenAI Codex assistance.