feat: add MDA-linked Stage Explorer overlays

[gcharvin]

Summary

Draft feature PR for design feedback. This adds an MDA-linked Stage Explorer variant that can:

• display positions from the MDA position table as stage-space overlays
• track selected and active MDA positions
• load a DXF chip overlay and align it to the microscope stage using a reference point
• include tests for MDA position tracking and DXF/ACIS geometry parsing

Background

This was developed from a TiEclipse workflow where we need a spatial overview of MDA positions and chip geometry while preparing acquisitions. I do not have screenshots saved yet, so this PR is documented from the commits and validation tests.

Notes for reviewers

This is intended as a discussion PR, not a request to merge immediately. In particular, I would like feedback on whether this belongs in pymmcore-gui, in pymmcore-widgets, or behind a more generic extension point.

Validation

• python -m ruff check pyproject.toml src/pymmcore_gui/_main_window.py src/pymmcore_gui/actions/widget_actions.py src/pymmcore_gui/widgets/_chip_dxf.py src/pymmcore_gui/widgets/_mda_stage_explorer.py tests/test_chip_dxf.py tests/test_mda_stage_explorer.py
• QT_QPA_PLATFORM=offscreen python -m pytest tests/test_chip_dxf.py tests/test_mda_stage_explorer.py -q: 8 passed
• python -m compileall -q src/pymmcore_gui/widgets/_chip_dxf.py src/pymmcore_gui/widgets/_mda_stage_explorer.py

[codecov]

Codecov Report

❌ Patch coverage is 81.03448% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.62%. Comparing base (76024ca) to head (11bf09a).

Files with missing lines	Patch %	Lines
src/pymmcore_gui/widgets/_mda_stage_explorer.py	83.23%	58 Missing ⚠️
src/pymmcore_gui/widgets/_chip_dxf.py	71.69%	30 Missing ⚠️

❌ Your patch check has failed because the patch coverage (81.03%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
- Coverage   81.84%   81.62%   -0.23%     
==========================================
  Files          29       31       +2     
  Lines        1807     2269     +462     
==========================================
+ Hits         1479     1852     +373     
- Misses        328      417      +89     

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[tlambert03]

screenshot: