feat: live view click-to-move fix and Ctrl+Scroll Z navigation

[Alpaca233]

Summary

• Centralize displayed-image pixel size in a new Microscope.get_image_pixel_size_um() helper and use it from move_from_click_image. X is now issued non-blocking immediately followed by a blocking Y so both axes finish before the call returns. Adds an info-level log line that names every factor (lens, binned_um, binning, pixel_size_um, mm delta) so the next distance regression can be diagnosed from a single click.
• Adds Ctrl+Scroll → 1 µm Z step and Ctrl+Shift+Scroll → 20 µm Z step on the ImageDisplayWindow live view via a Qt event filter. Plain scroll continues to zoom (event is not consumed). Z scroll is gated on the click-to-move checkbox (which toggleAcquisitionStart already forces off during acquisition) and the stage-busy flag, and clamped to SOFTWARE_POS_LIMIT.Z bounds.
• New step constants LIVE_VIEW_Z_STEP_UM = 1 and LIVE_VIEW_Z_STEP_FAST_UM = 20 in _def.py.

Out of scope: the napari live view widget. True simultaneous XY motion (firmware has no combined-move command).

Test plan

• New unit tests for Microscope.get_image_pixel_size_um() (returns product, returns None when either factor is missing).
• New unit tests for ImageDisplayWindow.eventFilter (Ctrl/Ctrl+Shift wheel emits ±1 / ±20 µm; zero notch consumed without emit; plain scroll passes through unchanged).
• Black format check passes.
• Full test suite passes (excluding pre-existing failures in tests/control/core/test_zarr_writer.py that are present on master).
• Manual on hardware: double-click recenters correctly across cameras (capture the new click_to_move: info log if distance is still off so we can identify the wrong factor).
• Manual on hardware: Ctrl+Scroll moves Z by 1 µm/notch, Ctrl+Shift+Scroll by 20 µm/notch; clamps at 0/6000 µm with a debug log.
• Manual on hardware: Z scroll is inert during acquisition and while the stage is busy.

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