feat: add experimental MDA position map widget

[gcharvin]

Summary

Draft feature PR for discussion. This adds an experimental MDA Position Map widget that gives a simple XY overview of positions defined in the MDA widget.

It includes:

• a design note describing the intended workflow
• a Qt graphics-view based overview of MDA position footprints
• table selection synchronization between the map and the MDA position table

Background

This came from the same TiEclipse workflow as the Stage Explorer overlay work: when many positions are defined in the MDA, it is useful to see their spatial arrangement before acquisition. I do not have screenshots saved, so this is documented from commit history and code behavior.

Notes for reviewers

This may overlap with or be superseded by the Stage Explorer overlay approach. I am opening it as a draft to decide whether a separate lightweight map widget is useful, or whether the feature should be folded into a single Stage Explorer workflow.

Validation

• python -m ruff check MDA_POSITION_MAP_DESIGN.md src/pymmcore_gui/actions/widget_actions.py src/pymmcore_gui/widgets/_mda_position_map.py
• python -m compileall -q src/pymmcore_gui/widgets/_mda_position_map.py src/pymmcore_gui/actions/widget_actions.py

[codecov]

Codecov Report

❌ Patch coverage is 88.66667% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.37%. Comparing base (76024ca) to head (11b9483).

Files with missing lines	Patch %	Lines
src/pymmcore_gui/widgets/_mda_position_map.py	88.11%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
+ Coverage   81.84%   82.37%   +0.52%     
==========================================
  Files          29       30       +1     
  Lines        1807     1957     +150     
==========================================
+ Hits         1479     1612     +133     
- Misses        328      345      +17     

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