Add comprehensive migration guide (docs/MIGRATION.md)

Documents every breaking-change migration applied to the codebase:
Python 2->3.8+, ABC metaclass fix, BundleType gotcha, cPickle->pickle,
integer division, pathlib, dV/dU dimension bugs, C++17, Boost removal,
C headers, data-race fixes, MuJoCo 1.31->3.x API + XML deprecated
attributes, TF1->TF2 compat shim, protobuf version matching, 3rdparty
CMake Python 2.7 removal, test suite setup, CI workflows, and known
remaining limitations.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: robotsorcerer