fix: skip focus-map interpolation for single-FOV snap acquisitions

[mpiersonsmela]

Summary

Fixes a crash when clicking Snap Images in Wellplate Multipoint mode while a per-well focus map is loaded.

Bug

File "control/widgets.py", line 8551, in on_snap_images
    self.multipointController.run_acquisition(acquire_current_fov=True)
File "control/core/multi_point_controller.py", line 781, in run_acquisition
    z = self.focus_map.interpolate(x, y, region_id)
File "control/core/core.py", line 1933, in interpolate
    raise ValueError(f"Region {region_id} not found")
ValueError: Region current not found

Steps to reproduce (on master)

1. Launch the GUI (main_hcs.py), switch to Wellplate Multipoint mode.
2. Select one or more wells and set focus points, then enable the focus map (fit_by_region=True — one surface per well). The focus map ends up keyed by well ID (B1, B2, …).
3. Position the stage over any point on the plate.
4. Select an imaging channel and click Snap Images.
5. → crash: ValueError: Region current not found.

Root cause

run_acquisition(acquire_current_fov=True) at multi_point_controller.py:722-735 creates a temporary ScanCoordinates with a single region literally named "current" for the one-off snap. Immediately after, the focus-map block (multi_point_controller.py:774-784) runs unconditionally:

if self.focus_map:
    for region_id in scan_position_information.scan_region_names:
        ...
        z = self.focus_map.interpolate(x, y, region_id)

With fit_by_region=True, FocusMap.interpolate requires region_id ∈ region_surface_fits (core.py:1931-1933). "current" is never in that dict, so the call raises.

Fix

Gate the focus-map block on not acquire_current_fov so it applies only to real multi-region acquisitions:

if self.focus_map and not acquire_current_fov:

Rationale: Snap Images is a manual snapshot. on_snap_images at widgets.py:8547-8548 already sets set_z_range(z, z) from stage.get_pos().z_mm — the user's currently focused Z. Overriding that Z with a focus-map interpolation was never the intended behavior for the single-FOV path, so skipping the block is the correct semantics and resolves the region-name mismatch.

How to verify the fix

1. On this branch, repeat steps 1-4 above → snap succeeds, image saved.
2. Regression check — regular wellplate multipoint acquisition (Start Acquisition, not Snap Images) with the focus map still applies Z interpolation per well as before. The log line Using focus surface for Z interpolation should still appear for full acquisitions, and NOT for Snap Images.
3. Snap Images without a focus map still works exactly as before.

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