Support .obsm backed color specs in val.viz.embedding

[JhonatanFelix]

Summary

Closes #55

This draft PR is meant to cover a small V1 for supporting .obsm-backed color specs in val.viz.embedding

Planned scope for this work:

• Parser / expansion

  Add private parsing and expansion helpers for color specs like foo[2] and bounded slices like foo[2:6] while keeping normal .obs keys unchanged.

• val.viz.embedding wrapper

  Replace the current direct re-export with a small wrapper that resolves .obsm vector specs, creates temporary .obs columns on a copy of adata, and then forwards the rewritten color argument to scanpy.pl.embedding.

• Tests and polish

  Add focused unit tests for the parsing and wrapper behavior and plus a small smoke test to make sure the feature works end-to-end without mutating the original adata.

For this V1, I’m intentionally keeping the scope pretty "small":

• support foo[i]
• support bounded slices like foo[a:b]
• allow mixing these with normal .obs color keys
• leave foo[:] out for now

Checklist

• CHANGELOG.md updated under [Unreleased] (required for user-facing changes)
• Tests added or updated if needed
• Docs updated if needed

[patcon]

thanks! will aim to review today, though maaaaybe tomorrow 🙏

[JhonatanFelix]

Hi @patcon, I wanted to follow up on this PR in case it got buried
Happy to make any adjustments if needed!