STARBackend: create 96-head mix96

[BioCam]

Creates mix96 - in-place mixing with the rigid 96-head, built on the head96_experimental_aspirate / head96_experimental_dispense primitives. It positions the head over a target and runs repeated surface-following aspirate / dispense cycles, taking the mix parameters directly rather than a liquid class. Builds on #1089 (the experimental aspirate / dispense), #1081 (the head96_move_x acceleration control), and #1086 / #1088 (the tip-bottom Z reference and per-drive speed / acceleration scoping).

  @_requires_head96
  @need_iswap_parked
  async def mix96(
    self,
    mix: Mix,
    resource: Optional[Union[Plate, Container, List[Well]]] = None,  # aspirate96-style target
    a1_coordinate: Optional[Coordinate] = None,        # or an explicit channel-A1 deck target
    minimum_traverse_height_start: Optional[float] = None,  # abs Z before X/Y; None -> Z-safety
    offset: Coordinate = Coordinate.zero(),            # added to the target; offset.z lifts the floor
    lld_mode: Optional[LLDMode] = None,                # None -> LLDMode.OFF (only OFF supported)
    descent_speed: float = 80.0,                       # mm/s, fast descent to just above the well
    swap_speed: float = 5.0,                           # mm/s, careful descent into the well and exit
    settling_time: float = 0.0,                        # s, wait after the last cycle before exit
    minimum_traverse_height_end: Optional[float] = None,    # abs Z after mixing; None -> Z-safety
  ):

The target is declared in exactly one of two ways, like aspirate96: an explicit a1_coordinate, or a resource (a Plate - head A1 over well A1 - a Container, or a list of Wells) plus offset. All Z is tip-bottom referenced: the declared Z (or a well's cavity_bottom) is the floor, the deepest the tip end reaches, and offset.z lifts the whole stroke off it.

Motion: parks the iSWAP, traverses to a start height, moves X/Y (X acceleration dialled down below y=200 mm, the low-Y zone where the head is cantilevered off the X-drive), descends in two stages (descent_speed to just above the well, then swap_speed into it), runs mix.repetitions symmetric cycles between the floor and surface_following_distance above it, waits settling_time, then retracts at swap_speed before the traverse out.

# explicit channel-A1 deck coordinate
await star.mix96(Mix(volume=50, repetitions=3, flow_rate=100), a1_coordinate=Coordinate(500, 150, 100))

# aspirate96-style resource target (head A1 over well A1)
await star.mix96(Mix(volume=50, repetitions=3, flow_rate=100), resource=plate)

lld_mode currently accepts only LLDMode.OFF; liquid-level-driven auto surface following (computing the surface-following distance from a cLLD probe) is planned as a follow-up.

Validated on hardware: the surface-following primitive this builds on moved the tip exactly 8 mm down and back, repeatably with no drift (#1089).

Tests: two minimal orchestration assertions for the highest-risk paths - the experimental command's minimum_height resolves to the tip-bottom floor (a well's cavity_bottom plus offset.z), and the careful descent starts surface following at floor + surface_following_distance so the stroke spans [floor, floor+sf] and never drives below the floor.