feat(solver): KCSolve solver addon with assembly integration (#289)

Adds the Kindred constraint solver as a pluggable Assembly workbench
backend, covering phases 3d through 5 of the solver roadmap.

Phase 3d: SolveContext packing
- Pack/unpack SolveContext into .kc archive on document save

Solver addon (mods/solver):
- Phase 1: Expression DAG, Newton-Raphson + BFGS, 3 basic constraints
- Phase 2: Full constraint vocabulary — all 24 BaseJointKind types
- Phase 3: Graph decomposition for cluster-by-cluster solving
- Phase 4: Per-entity DOF diagnostics, overconstrained detection,
  half-space preference tracking, minimum-movement weighting
- Phase 5: _build_system extraction, diagnose(), drag protocol,
  joint limits warning

Assembly workbench integration:
- Preference-driven solver selection (reads Mod/Assembly/Solver param)
- Solver backend combo box in Assembly preferences UI
- resetSolver() on AssemblyObject for live preference switching
- Integration tests (TestKindredSolverIntegration.py)
- In-client console test script (console_test_phase5.py)

.gitmodules

@@ -18,3 +18,7 @@
 	path = mods/silo
 	url = https://git.kindred-systems.com/kindred/silo-mod.git
 	branch = main
+[submodule "mods/solver"]
+	path = mods/solver
+	url = https://git.kindred-systems.com/kindred/solver.git
+	branch = main