[sjdoane]

The default binary ANSYS writer raises KeyError: dtype('uint64') for unsigned mesh connectivity, as reported in #1335. This affects supported triangle/tetra cells produced by tools such as pygmsh.

Convert unsigned connectivity into a fresh native int64 array before selecting the existing signed64 binary section. Check that the one-based node IDs fit first, avoiding narrowing or wraparound. Signed and ASCII paths stay unchanged, and input arrays are not modified.

Validation on Python 3.12 / NumPy 2.5.4, macOS arm64:

• New regressions: 12 failures / 11 passes before; 23 passes after. Combined with the existing ANSYS tests: 41 passes, independently repeated in a separate checkout.
• An actual pygmsh/Gmsh-generated uint64 triangle surface fails on base and round-trips geometry/connectivity after the fix. Mixed cells, unsigned widths, endianness, read-only inputs, signed controls and binary wire/range boundaries are checked; the reviewer adds 32 independent cases.
• Changed-file lint, configured Black/isort and diff checks pass. The broader suite remains failing: 686 passed / 75 failed / 10 skipped / 2 errors; the same 77 pre-existing failure/error IDs occur on base, including missing LFS fixtures and existing NumPy 2 formatting failures. The reviewer did not rerun that entire baseline comparison.

This addresses the dtype defect for existing ANSYS-supported cells. The unfiltered example in #1335 also contains already-unsupported vertex/line cells; this change does not add those. Solver import, other platform matrices and hosted CI were not tested locally.

Implementation and independent local review were performed by separate Codex agents.