feat: make capture_ids optional with 96-well-plate auto-generation

[thomas-enzlein]

Summary

Makes --cap_ids optional for both csv and xml formats. When omitted, capture IDs are auto-generated using 96-well-plate naming (A1, B1, ..., H12, A13, ...).

Changes

utils.py (new)

• Shared generate_wellplate_ids(n) — generates well-plate-style IDs, used by both CSV and XML modules

shape2xml_v2.py

• capture_ids parameter now defaults to None
• When None: auto-generates well-plate IDs via generate_wellplate_ids()
• Old local _generate_wellplate_ids removed in favor of shared utility

shape2csv_v2.py

• Added optional capture_ids parameter (default None)
• Segment-group comment changed from bare # to # Group: <cap_id>
• Auto-generates well-plate IDs when capture_ids is None
• Same treatment for addshape2csv()

main.py

• Cap-ID-to-label mapping extracted before format dispatch, shared by CSV and XML
• CSV branch now passes capture_ids=mapped_cap_ids
• --cap_ids help text updated to reflect optional nature

.github/workflows/docker-build-pr.yml (new)

• CI workflow that builds the Docker image on every PR to master
• Build only (no push) — verifies the image compiles successfully

Testing

• PR CI automatically builds the Docker container
• Run locally: docker build -t shape2instrument:test . && docker run --rm shape2instrument:test --help

Closes: # (issue if applicable)