Fix restart float test and vcpkg Windows CI

[zayenz]

This fixes two independent failures exposed by the current test matrix.

The on_restart::sol_float fixture left its output float unbounded even though the expected first solution assumes a lower bound of 1.0. Restart initialization uses the variable lower bound when no previous solution exists, so the test printed Gecode’s minimum representable float. The output now uses the same 1.0..3.0 domain as the test input and restart variables.

The Windows vcpkg job was pinned before microsoft/vcpkg#53437, where the GMP port still referenced an unavailable Autoconf 2.71 package. The workflow checkout, cache key, and manifest baseline now use vcpkg commit 37bb045f3c7a747d3e5d1c13b6fe6a0aec4b5d00. That baseline selects GMP 6.3.0#5 and its reachable autoconf2.71-2.71-4 package. GMP intentionally remains on Autoconf 2.71 because its port documents broken dumpbin detection with 2.72.

Validation:

• reproduced the original FlatZinc::on_restart::sol_float failure on macOS
• rebuilt gecode-test natively
• exact restart-float test passes for all five default iterations
• all 12 FlatZinc::on_restart::* tests pass
• vcpkg.json and the workflow parse successfully
• downloaded the pinned MSYS2 package and matched its SHA-512
• diff hygiene passes