[timothyanderson096-ocdealcheck]

Problem
geo2grid() can select UTM zone 61 at +180 degrees longitude and immediately below it due to floating-point rounding. UTM has only zones 1–60. grid2geo() rejects the resulting invalid zone.
Scoped correction
Clamp only the automatically computed zone to 60 at the upper boundary. Preserve the conventional -180 -> zone 1 and +180 -> zone 60 assignments, explicit zone selection, longitude validation, and ISG behaviour. No change to the separate vincdir() fix in #174 or decimal-year support in #175.
Tests and evidence

• Original code: 16 regression cases failed; 29 passed.
• Corrected code: 119/119 passed (74 existing + 45 regression cases).
• Verified GitHub Actions: https://github.com/timothyanderson096-ocdealcheck/GeodePy/actions/runs/39535947460
• Independent PROJ comparison: 36 latitude/longitude cases across both boundary representations agreed within 0.000152 metres.
• 2,000 additional ordinary-location zone-consistency checks passed locally.
  Numerical tests do not prove physical positioning accuracy. Boundary representation is offered for maintainer review.

Prepared by Tim Anderson / OC Labs with AI-assisted testing.