ENH: model unbonded solid-motor grain CM shift (#340)

[thatrandomasiandev]

Summary

• Adds opt-in SolidMotor(grains_bonded=True) (default preserves current fixed-grain / BATES behavior).
• When grains_bonded=False, applies a first-order inertial packing model: grains pack against the aft (nozzle-side) face of the initial grain stack, so propellant CM shifts toward the nozzle as grain_height regresses: CM(t) = grains_center_of_mass_position - _csys * (n/2) * (h0 - h(t)). Parallel-axis propellant_I_11 uses the packed pitch.
• Documents that this is not a DEM / rigid-body grain dynamics model (no friction, rattling, or acceleration threshold). Inter-grain grain_separation (spacers) is retained; only grain-height loss shortens the stack. With only_radial_burn=True, height is fixed so CM does not shift.

Fixes #340

Follow-ups (out of scope)

• Acceleration-dependent settling / ignition transient before packing completes
• Full multi-body / DEM grain contact model
• Optional collapse of soft spacers (grain_separation → 0 when packing)

Test plan

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest tests/unit/motors/test_solidmotor.py
• Bonded path matches prior fixed CM
• Unbonded multi-grain CM moves aft vs bonded mid-burn; ignition CM matches
• grains_bonded round-trips through to_dict / from_dict