Prep dispense: three layers, batched as the aspirate, in the STAR's names and order

[BioCam]

Main's Prep dispense is one method that resolves and sends everything itself, with its own argument names (vols, liquid_height, z_final, z_liquid_exit_speed, cutoff_speed), while the aspirate is already three layers in the STAR's names and order (#1371). It also sends every channel sharing a container to its centre.

• _unchecked_fw_dispense sends the structs as given; _dispense_in_one_move builds one command from positions and heights; dispense plans batches in ascending X, as aspirate, and refuses everything the model can decide before the first move.
• Arguments renamed and ordered as the STAR's: volumes or piston_volumes, liquid_heights, immersion_depths, minimum_allowed_z_positions_during, cut_off_speeds, stop_back_volumes, settling_times, swap_speeds, minimum_traverse_height_end.
• The STAR's fallbacks: flow rate 120.0 uL/s and cut-off speed 5.0 where no liquid class gives them.
• The STAR's side_touch_off_distance, post_mixes / mix_positions_from_liquid_surface and limit_curve_indices slots are taken and refused: the Prep has no side touch-off, and post-mixing and TADM are not verified.
• Channels sharing a container spread across it in Y, as on aspirate.
• The demo notebook's dispense call uses the new names.

Tests: pipettes_tests - golden frames for every dispense variant, one command per X batch, refusals before booking or sending, immersion_depths, the refused slots. _aspirate_in_one_move's keyword order is pinned to the STAR's; main's STAR has no dispense to pin against yet.

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