Implement improvement plan (phases 0-8): agent-native creature workbench

[iodriller]

Implements all 8 phases of docs/IMPROVEMENT_PLAN.md, turning Creature Lab from a single stationary tripod demo into an agent-native articulated-creature workbench. Every behavioural claim was verified against the live PyBullet/MuJoCo backends.

What's in it

• Phase 0 — Demo UX. Diagnosed the real "stationary creature" cause (1 m spawn crash-land + a no-thrust tripod gait). Default demo is now a forward-walking quadruped (+0.58 m); added a worm; demo --creature {quadruped,worm,tripod}.
• Phase 1 — Scaffolds. scaffold/ generators (worm, quadruped, hexapod, humanoid) + mirror_limb; export-urdf (verified loadable in PyBullet).
• Phase 2 — Creature Zoo. Curated gallery (zoo.py + zoo/ data) with computed baselines; zoo list|run|validate-all.
• Phase 3 — Failure diagnosis. diagnosis.py with 8 locomotion failure patterns; diagnose explains why a run failed and suggests fixes.
• Phase 4 — Closed-loop control. ObservationSpec/ActionSpec, a Gymnasium-style CreatureEnv, and CPG/PID/pose-sequence controllers; run --controller {sinusoid,cpg}.
• Phase 5 — Debug viewer. CoM/root trails + fall marker overlays, side-by-side compare, and matplotlib plot.
• Phase 6 — Humanoid kit. humanoid_minimal/humanoid_12dof zoo entries, impulse events for push-recovery, and 4 biped-specific diagnosis patterns.
• Phase 7 — Evolution. Typed mutation operators + crossover and four strategies (hill_climb, genetic, map_elites, cmaes); lineage tree; archive/lineage JSON.
• Phase 8 — Bridges. Full MJCF export + a MuJoCo backend, URDF <transmission> blocks, and best-effort URDF import; export-mjcf, import-urdf, run --backend mujoco.

Re-audit fixes (post-implementation review)

• Control-mode bug: CreatureEnv velocity/torque actions were silently applied as position targets, and the env could not drive hinges lacking a MotorSpec. Added PyBulletBackend.apply_joint_control (drives any hinge by id in position/velocity/torque); the env now uses it. MuJoCo supports position only (raises clearly otherwise). Regression tests added.
• CI: now installs the evolve + mujoco extras so CMA-ES and the MuJoCo backend are actually exercised (previously always skipped).
• Housekeeping: gitignore MUJOCO_LOG.TXT and *.urdf; fixed a FutureWarning/latent bug in URDF import (childless-element truthiness).

Notable judgments (documented in the plan)

• MJCF maps motors to <position> servos (not plain <motor>) so the exported model is actually drivable; added joint damping/armature + implicitfast integrator for solver stability.
• Per the project's portability promise, PyBullet-tuned gaits don't reproduce their exact motion under MuJoCo — the backend runs stably and scores finitely, which is the contract.
• Evolution lineage is stored as a self-contained lineage.json rather than new TraceMeta fields.

Tests

232 passing (from 132), ruff check + format clean. MuJoCo/CMA-ES tests skip gracefully when their extras are absent.