Add STARBackend.channels_probe_z_using_ztouch for parallel force probing

[BioCam]

The Problem

PyLabRobot has a single-channel force-sensed Z probe (ztouch_probe_z_height_using_channel, firmware ZH) but no batched, parallel equivalent. Multi-channel workflows that need this measurement currently require careful and time-consuming planning.

Concrete needs that surface this gap:

• Verifying that lids, adapters, or labware are seated at the expected Z before a workflow continues.
• Calibrating the Z of unknown or replaceable deck features without manual teach-in.
• Capturing per-channel Z offsets against a calibration block in one shot rather than N sequential probes.

Naively wrapping the primitive in asyncio.gather does not solve this, because of two parallelism hazards specific to force-sensed probing on the STAR head:

• Carriage-force cross-talk. All channels share one rigid Z-drive carriage. When channels descend simultaneously, each channel's touchdown produces a force transient that perturbs the PWM-based detection threshold on the others, biasing every measurement after the first to land.

• C0-command serialization. The single-channel primitive issues move_channel_z (firmware JZ via C0) when post_detection_dist != 0 (TODO: move to PX version), and request_tip_len_on_channel (also C0) when tip_len=None. Both route through the master controller and serialize the gather - defeating the parallelism the caller is trying to get.

A correct batched implementation has to address both of these explicitly.

PR Content / Solution

Adds channels_probe_z_using_ztouch to the STAR backend: a batched, parallel force-sensed Z probe that returns the absolute deck-frame Z of whatever solid surface each channel first touches.

Architecture

Reuses the same batched-channel orchestration skeleton that probe_liquid_heights already uses for cLLD/pLLD: plan_batches builds channel/X/Y batches, execute_batched handles inter-batch X/Y positioning and Z safety, and the per-batch callback fans out ztouch_probe_z_height_using_channel across the batch's channels under asyncio.gather. No reinvention of X/Y partitioning, no-go-zone handling, or inter-batch Z safety.

Carriage cross-talk: stagger the channel starts

Within each batch, channel starts are staggered by inter_channel_start_delay (default 0.3 s) so contact-force transients don't superpose. The stagger lives in a small _delayed(delay, factory) helper that takes a coroutine factory (not a coroutine) to avoid un-awaited-coroutine warnings if the gather is cancelled mid-sleep. Tunable to 0.0 for callers who've validated mechanically isolated geometry.

C0-avoidance: keep the gather truly parallel

The inner ZH call receives post_detection_dist=0 and an explicit tip_len so the primitive does not issue move_channel_z or request_tip_len_on_channel. Channel raising between batches and at the end is handled by the existing min_traverse_height_during_command and z_position_at_end_of_command kwargs.

Safety and aggregation

Outer try / except BaseException lifts all channels to Z safety on any exception (firmware error, KeyboardInterrupt, SystemExit, asyncio.CancelledError) before re-raising. Aggregated Z is rounded to the firmware z-drive quantum (0.01 mm) so users don't see float-averaging noise across replicates.

Design decisions

• Returns absolute deck-frame Z, not relative to a container or any other reference. The method targets arbitrary surfaces; there's no "in this well" frame to subtract. Callers wanting relative values subtract themselves.
• No public post_detection_dist kwarg. Using it inside the gather would issue a C0 JZ per channel and serialize the parallelism. Channel raising is already covered by min_traverse_height_during_command / z_position_at_end_of_command.
• except BaseException, not except Exception. Catches KeyboardInterrupt, SystemExit, and asyncio.CancelledError too - all cases where channels might be pressing on a surface when control is lost. Matches the pattern already in execute_batched.
• inter_channel_start_delay default 0.3 s. Empirical default covering a typical ~tens-of-ms contact transient with margin; tunable to 0.0 for callers who don't need the decoupling.
• Round aggregated Z to 0.01 mm.