fix: first axis should be the fast axis

[wakonig]

Description

This PR fixes a bug in particular for the grid scan: The fast axis should be the first one, not the inner one.

Type of Change

• Make the first axis always the fast one. This is much easier to communicate and easier to understand when using the GUI.

How to test

• Run unit tests
• Plot the positions with BW

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR changes scan position generation so the first user-provided axis is always the fast axis (innermost loop), aligning scan traversal with GUI/user expectations—particularly for grid-like scans. It updates both the v4 scan position generators and the legacy scan implementations, and adjusts/extends unit tests to validate the new ordering.

Changes:

• Make the first axis the fast axis for N-D raster/grid position generation (v4 + legacy).
• Update multi-region grid traversal/snaking semantics to snake the first axis within each sub-grid.
• Update existing tests and add new assertions for grid and hexagonal scans to enforce the new traversal order.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
bec_server/bec_server/scan_server/scans/position_generators.py	Updates grid/multi-region/hex position generation so axis 0 is fast; adjusts snaking semantics/documentation.
bec_server/bec_server/scan_server/scans/legacy_scans.py	Mirrors the fast-axis behavior change for legacy raster and hex grid generators.
bec_server/bec_server/scan_server/scans/multi_region_grid_scan.py	Updates docstring to match new “snake first axis” semantics.
bec_server/tests/tests_scan_server/test_legacy_scans.py	Updates expected legacy grid/raster traversal ordering and corresponding device set sequence.
bec_server/tests/tests_scan_server/scans_v4/test_position_generators.py	Updates expected multi-region grid ordering.
bec_server/tests/tests_scan_server/scans_v4/test_multi_region_grid_scan.py	Updates expected prepared scan positions (absolute + relative offset cases).
bec_server/tests/tests_scan_server/scans_v4/test_hexagonal_scan.py	Adds explicit test that the first axis is treated as fast for hex scans.
bec_server/tests/tests_scan_server/scans_v4/test_grid_scan.py	Adds explicit test that the first axis is treated as fast for grid scans.

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