Fix stiff rod corrections with multiple static segments - #207

The direct stiff rod solver stops writing position and orientation corrections when its forward traversal reaches a static segment. In a three-segment rod with both ends fixed, the traversal visits the static leaf first, so the middle dynamic segment receives no correction.

Skip static segments and continue updating the remaining segments. Add an opt-in CTest regression using the public solver API, covering translation and twist with all segments dynamic, one fixed end, and two fixed ends.

Fixes #79.

Validation:

• Before the fix, both two-fixed-end cases fail; the other four cases pass.
• After the fix, all six cases pass in four MinGW GCC 15.2 configurations: float/double crossed with checked Debug (-O0, libstdc++ debug assertions) and Release (-O3 -DNDEBUG).
• Static segment correction outputs remain unchanged.
• CMake 3.31.8 builds the core library and regression target; CTest passes 1/1 tests (six internal cases). Explicitly disabling tests and leaving the option unset both produce no test target or registered tests.

Validation scope: Windows MinGW. The full interactive demo build, MSVC and Linux/macOS builds have not been run.