Fix GraphPlan: correct mutexes, interference, and level-off detection

[dmeoli]

Reworked, cleaned-up version of the GraphPlan fix originally proposed by @carwyn987 in #1315 (authorship of the commit is preserved). It keeps the valuable algorithmic work and adapts it to the current master while following the repo's conventions.

What it does

• Reworks the GraphPlan engine: explicit proposition vs action mutexes, interference handling, proposition-level mutex propagation, invalid-action pruning, memoized backward solution extraction, and stronger level-off detection (state + mutex stability).
• Adds a comprehensive tests/test_graphplan.py.
• Adds curated planning environments: logistics_problem, generalized blocks_world, rush_hour, rush_hour_optimized.

Differences from #1315 (kept it in AIMA style)

• Planning environments stay in planning.py (no separate planning_envs.py); from planning import * keeps working for the notebooks.
• predicate_negate is a module-level helper in planning.py (the core Expr class in utils.py is untouched).
• snake_case names and triple-quoted docstrings throughout; PEP 8 restored.
• Dropped the experimental double_tennis_problem_simple* variants; double_tennis_problem/PartialOrderPlanner left unchanged.
• requirements.txt keeps pytest-cov; README.md unchanged.

Tests

• tests/test_graphplan.py: 34 passed
• tests/test_planning.py: 37 passed
• planning.py doctests: 102 passed

Supersedes #1315.