Preserve Gist mode with statistics

[zayenz]

FlatZinc option parsing changed the execution mode to stat whenever -s was present, even when -mode gist had been selected explicitly. Argument order did not help because the override happened after parsing.

Keep -s as the statistics shorthand for ordinary solution mode, but preserve an explicit Gist mode in either argument order.

When Gist support is compiled out, reject Gist execution before search with a clear FlatZinc error instead of silently falling through to ordinary solution mode. The run-level regression parses and prepares a real model, then verifies that -mode gist -s raises the unavailable-mode error without producing a solution.

The focused FlatZinc option tests are included in both the CMake and Autotools fast-check selections.

Checks:

• Gist-disabled CMake Release build of fzn-gecode and gecode-test
• FlatZinc::Options::GistStatisticsMode
• FlatZinc::Options::GistUnavailable
• Gist-disabled fzn-gecode -mode gist -s process exits 1 with the expected error
• Gist-enabled CMake Release build of fzn-gecode
• git diff --check

Closes #99.