fix: implement GE -1 zero-current ground mode and signed-mode validation

[tmolteno]

Closes #133.

Summary

The zero-current ground mode (NEC GE -1) was documented in the C API but effectively unimplemented: build_connections() guarded every plane check with ignd > 0, so a GE -1 geometry skipped the below-plane / in-plane rejections the NEC-2 Part 3 manual mandates, and a GE ±1 with no GN card silently simulated free space.

This PR makes the code match the reported behavior:

• Public ABI: new nec_ground_connection enum — NONE = 0, IMAGE = 1, ZERO_CURRENT = 2. The stable public value for zero-current remains 2 while the native geometry receives the signed NEC value -1 (mapped in the C wrapper).
• Validation: both signed ground modes now reject segments that extend below the ground plane or lie in it; segments may still end on the plane. GE -1 keeps the Fortran connection semantics — a ground-touching end stays a free end, so the current goes to zero there — and the low-horizontal-wire escape hatch (height < 1e-3 × segment length) still applies, so only a segment with both ends exactly on the plane is rejected as lying in it.
• Ground model required: a non-none connection without a GN card now fails at simulate() (NEC-2 Part 3: "A positive or negative value of I1 does not cause a ground to be included in the calculation... The ground parameters must be specified on a program control card following the geometry cards.") instead of silently running free space.

Behavior changes for existing inputs

• testharness/data/discone.nec used GE 1 with no GN card — gained GN 1.
• testharness/python/test_examples.py::test_example1 completed the geometry with flag 1 but no ground model (its own deck comment is plain GE) — now uses 0.

Everything else is behavior-preserving: GE 0 and GE 1 outputs are byte-identical to master on all testharness decks (only the build-date header line differs).

Verification

• Unit suite: 708 assertions in 79 test cases, all pass, including 7 new [ground] test cases:
  • both signed modes reject below-plane and in-plane segments;
  • GE -1 accepts a segment ending on the plane and a low horizontal wire;
  • nec_geometry_complete(nec, 2) maps to native -1 (rejects a below-plane segment, records ground_connection() == -1);
  • ground connection without a GN card throws at xq_card(); with GN 1 it simulates.
• All 41 runnable testharness decks produce output identical to master (the 11 that fail on master fail identically here — pre-existing overlap-check failures, unrelated to this change).
• example/test_nec.c compiles clean and runs with the expected impedance values.