Add mods/solver/ submodule pointing to kindred/solver.git which contains
the Phase 1 expression-based Newton-Raphson constraint solver implementing
the IKCSolver interface via kcsolve.register_solver().
Phase 1 supports: Coincident, DistancePointPoint, Fixed constraints.
Also adds CMake install rules for the solver addon.
Add branch = main to mods/silo and mods/ztools in .gitmodules.
Enables 'git submodule update --remote' to auto-advance to latest main.
Third-party deps (GSL, googletest, AddonManager) remain pinned.
Syncing and update submodule should work, but if it doesn't then the following will reset it (+ remove all local changes):
```sh
git submodule deinit -f src/3rdParty/OndselSolver
git submodule update --init --recursive src/3rdParty/OndselSolver
```
